| 1 | #ifndef __LINUX_SEQLOCK_H
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| 2 | #define __LINUX_SEQLOCK_H
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| 3 | |
| 4 | * Reader/writer consistent mechanism without starving writers. This type of
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| 5 | * lock for data where the reader wants a consistent set of information
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| 6 | * and is willing to retry if the information changes. There are two types
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| 7 | * of readers:
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| 8 | * 1. Sequence readers which never block a writer but they may have to retry
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| 9 | * if a writer is in progress by detecting change in sequence number.
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| 10 | * Writers do not wait for a sequence reader.
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| 11 | * 2. Locking readers which will wait if a writer or another locking reader
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| 12 | * is in progress. A locking reader in progress will also block a writer
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| 13 | * from going forward. Unlike the regular rwlock, the read lock here is
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| 14 | * exclusive so that only one locking reader can get it.
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| 15 | *
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| 16 | * This is not as cache friendly as brlock. Also, this may not work well
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| 17 | * for data that contains pointers, because any writer could
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| 18 | * invalidate a pointer that a reader was following.
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| 19 | *
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| 20 | * Expected non-blocking reader usage:
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| 21 | * do {
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| 22 | * seq = read_seqbegin(&foo);
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| 23 | * ...
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| 24 | * } while (read_seqretry(&foo, seq));
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| 25 | *
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| 26 | *
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| 27 | * On non-SMP the spin locks disappear but the writer still needs
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| 28 | * to increment the sequence variables because an interrupt routine could
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| 29 | * change the state of the data.
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| 30 | *
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| 31 | * Based on x86_64 vsyscall gettimeofday
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| 32 | * by Keith Owens and Andrea Arcangeli
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| 33 |
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| 34 |
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| 35 | #include <linux/spinlock.h>
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| 36 | #include <linux/preempt.h>
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| 37 | #include <linux/lockdep.h>
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| 38 | #include <linux/compiler.h>
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| 39 | #include <asm/processor.h>
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| 40 |
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| 41 | |
| 42 | * Version using sequence counter only.
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| 43 | * This can be used when code has its own mutex protecting the
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| 44 | * updating starting before the write_seqcountbeqin() and ending
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| 45 | * after the write_seqcount_end().
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| 46 |
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| 47 | typedef struct seqcount {
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| 48 | unsigned sequence;
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| 49 | #ifdef CONFIG_DEBUG_LOCK_ALLOC
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| 50 | struct lockdep_map dep_map;
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| 51 | #endif
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| 52 | } seqcount_t;
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| 53 |
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| 54 | static inline void __seqcount_init(seqcount_t *s, const char *name,
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| 55 | struct lock_class_key *key)
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| 56 | {
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| 57 | |
| 58 | * Make sure we are not reinitializing a held lock:
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| 59 |
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| 60 | lockdep_init_map(&s->dep_map, name, key, 0);
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| 61 | s->sequence = 0;
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| 62 | }
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| 63 |
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| 64 | #ifdef CONFIG_DEBUG_LOCK_ALLOC
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| 65 | # define SEQCOUNT_DEP_MAP_INIT(lockname) \
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| 66 | .dep_map = { .name = #lockname } \
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| 67 |
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| 68 | # define seqcount_init(s) \
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| 69 | do { \
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| 70 | static struct lock_class_key __key; \
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| 71 | __seqcount_init((s), #s, &__key); \
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| 72 | } while (0)
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| 73 |
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| 74 | static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
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| 75 | {
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| 76 | seqcount_t *l = (seqcount_t *)s;
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| 77 | unsigned long flags;
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| 78 |
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| 79 | local_irq_save(flags);
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| 80 | seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
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| 81 | seqcount_release(&l->dep_map, 1, _RET_IP_);
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| 82 | local_irq_restore(flags);
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| 83 | }
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| 84 |
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| 85 | #else
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| 86 | # define SEQCOUNT_DEP_MAP_INIT(lockname)
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| 87 | # define seqcount_init(s) __seqcount_init(s, NULL, NULL)
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| 88 | # define seqcount_lockdep_reader_access(x)
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| 89 | #endif
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| 90 |
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| 91 | #define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
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| 92 |
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| 93 |
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| 94 | |
| 95 | * __read_seqcount_begin - begin a seq-read critical section (without barrier)
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| 96 | * @s: pointer to seqcount_t
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| 97 | * Returns: count to be passed to read_seqcount_retry
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| 98 | *
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| 99 | * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
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| 100 | * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
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| 101 | * provided before actually loading any of the variables that are to be
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| 102 | * protected in this critical section.
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| 103 | *
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| 104 | * Use carefully, only in critical code, and comment how the barrier is
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| 105 | * provided.
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| 106 |
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| 107 | static inline unsigned __read_seqcount_begin(const seqcount_t *s)
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| 108 | {
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| 109 | unsigned ret;
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| 110 |
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| 111 | repeat:
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| 112 | ret = READ_ONCE(s->sequence);
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| 113 | if (unlikely(ret & 1)) {
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| 114 | cpu_relax();
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| 115 | goto repeat;
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| 116 | }
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| 117 | return ret;
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| 118 | }
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| 119 |
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| 120 | |
| 121 | * raw_read_seqcount - Read the raw seqcount
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| 122 | * @s: pointer to seqcount_t
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| 123 | * Returns: count to be passed to read_seqcount_retry
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| 124 | *
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| 125 | * raw_read_seqcount opens a read critical section of the given
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| 126 | * seqcount without any lockdep checking and without checking or
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| 127 | * masking the LSB. Calling code is responsible for handling that.
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| 128 |
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| 129 | static inline unsigned raw_read_seqcount(const seqcount_t *s)
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| 130 | {
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| 131 | unsigned ret = READ_ONCE(s->sequence);
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| 132 | smp_rmb();
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| 133 | return ret;
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| 134 | }
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| 135 |
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| 136 | |
| 137 | * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
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| 138 | * @s: pointer to seqcount_t
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| 139 | * Returns: count to be passed to read_seqcount_retry
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| 140 | *
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| 141 | * raw_read_seqcount_begin opens a read critical section of the given
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| 142 | * seqcount, but without any lockdep checking. Validity of the critical
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| 143 | * section is tested by checking read_seqcount_retry function.
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| 144 |
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| 145 | static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
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| 146 | {
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| 147 | unsigned ret = __read_seqcount_begin(s);
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| 148 | smp_rmb();
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| 149 | return ret;
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| 150 | }
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| 151 |
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| 152 | |
| 153 | * read_seqcount_begin - begin a seq-read critical section
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| 154 | * @s: pointer to seqcount_t
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| 155 | * Returns: count to be passed to read_seqcount_retry
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| 156 | *
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| 157 | * read_seqcount_begin opens a read critical section of the given seqcount.
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| 158 | * Validity of the critical section is tested by checking read_seqcount_retry
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| 159 | * function.
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| 160 |
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| 161 | static inline unsigned read_seqcount_begin(const seqcount_t *s)
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| 162 | {
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| 163 | seqcount_lockdep_reader_access(s);
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| 164 | return raw_read_seqcount_begin(s);
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| 165 | }
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| 166 |
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| 167 | |
| 168 | * raw_seqcount_begin - begin a seq-read critical section
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| 169 | * @s: pointer to seqcount_t
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| 170 | * Returns: count to be passed to read_seqcount_retry
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| 171 | *
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| 172 | * raw_seqcount_begin opens a read critical section of the given seqcount.
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| 173 | * Validity of the critical section is tested by checking read_seqcount_retry
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| 174 | * function.
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| 175 | *
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| 176 | * Unlike read_seqcount_begin(), this function will not wait for the count
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| 177 | * to stabilize. If a writer is active when we begin, we will fail the
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| 178 | * read_seqcount_retry() instead of stabilizing at the beginning of the
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| 179 | * critical section.
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| 180 |
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| 181 | static inline unsigned raw_seqcount_begin(const seqcount_t *s)
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| 182 | {
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| 183 | unsigned ret = READ_ONCE(s->sequence);
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| 184 | smp_rmb();
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| 185 | return ret & ~1;
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| 186 | }
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| 187 |
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| 188 | |
| 189 | * __read_seqcount_retry - end a seq-read critical section (without barrier)
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| 190 | * @s: pointer to seqcount_t
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| 191 | * @start: count, from read_seqcount_begin
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| 192 | * Returns: 1 if retry is required, else 0
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| 193 | *
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| 194 | * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
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| 195 | * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
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| 196 | * provided before actually loading any of the variables that are to be
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| 197 | * protected in this critical section.
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| 198 | *
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| 199 | * Use carefully, only in critical code, and comment how the barrier is
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| 200 | * provided.
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| 201 |
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| 202 | static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
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| 203 | {
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| 204 | return unlikely(s->sequence != start);
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| 205 | }
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| 206 |
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| 207 | |
| 208 | * read_seqcount_retry - end a seq-read critical section
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| 209 | * @s: pointer to seqcount_t
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| 210 | * @start: count, from read_seqcount_begin
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| 211 | * Returns: 1 if retry is required, else 0
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| 212 | *
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| 213 | * read_seqcount_retry closes a read critical section of the given seqcount.
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| 214 | * If the critical section was invalid, it must be ignored (and typically
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| 215 | * retried).
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| 216 |
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| 217 | static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
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| 218 | {
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| 219 | smp_rmb();
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| 220 | return __read_seqcount_retry(s, start);
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| 221 | }
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| 222 |
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| 223 |
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| 224 |
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| 225 | static inline void raw_write_seqcount_begin(seqcount_t *s)
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| 226 | {
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| 227 | s->sequence++;
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| 228 | smp_wmb();
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| 229 | }
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| 230 |
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| 231 | static inline void raw_write_seqcount_end(seqcount_t *s)
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| 232 | {
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| 233 | smp_wmb();
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| 234 | s->sequence++;
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| 235 | }
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| 236 |
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| 237 | |
| 238 | * raw_write_seqcount_barrier - do a seq write barrier
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| 239 | * @s: pointer to seqcount_t
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| 240 | *
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| 241 | * This can be used to provide an ordering guarantee instead of the
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| 242 | * usual consistency guarantee. It is one wmb cheaper, because we can
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| 243 | * collapse the two back-to-back wmb()s.
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| 244 | *
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| 245 | * seqcount_t seq;
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| 246 | * bool X = true, Y = false;
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| 247 | *
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| 248 | * void read(void)
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| 249 | * {
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| 250 | * bool x, y;
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| 251 | *
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| 252 | * do {
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| 253 | * int s = read_seqcount_begin(&seq);
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| 254 | *
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| 255 | * x = X; y = Y;
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| 256 | *
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| 257 | * } while (read_seqcount_retry(&seq, s));
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| 258 | *
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| 259 | * BUG_ON(!x && !y);
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| 260 | * }
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| 261 | *
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| 262 | * void write(void)
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| 263 | * {
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| 264 | * Y = true;
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| 265 | *
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| 266 | * raw_write_seqcount_barrier(seq);
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| 267 | *
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| 268 | * X = false;
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| 269 | * }
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| 270 |
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| 271 | static inline void raw_write_seqcount_barrier(seqcount_t *s)
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| 272 | {
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| 273 | s->sequence++;
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| 274 | smp_wmb();
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| 275 | s->sequence++;
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| 276 | }
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| 277 |
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| 278 | static inline int raw_read_seqcount_latch(seqcount_t *s)
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| 279 | {
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| 280 | int seq = READ_ONCE(s->sequence);
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| 281 |
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| 282 | smp_read_barrier_depends();
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| 283 | return seq;
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| 284 | }
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| 285 |
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| 286 | |
| 287 | * raw_write_seqcount_latch - redirect readers to even/odd copy
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| 288 | * @s: pointer to seqcount_t
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| 289 | *
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| 290 | * The latch technique is a multiversion concurrency control method that allows
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| 291 | * queries during non-atomic modifications. If you can guarantee queries never
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| 292 | * interrupt the modification -- e.g. the concurrency is strictly between CPUs
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| 293 | * -- you most likely do not need this.
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| 294 | *
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| 295 | * Where the traditional RCU/lockless data structures rely on atomic
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| 296 | * modifications to ensure queries observe either the old or the new state the
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| 297 | * latch allows the same for non-atomic updates. The trade-off is doubling the
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| 298 | * cost of storage; we have to maintain two copies of the entire data
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| 299 | * structure.
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| 300 | *
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| 301 | * Very simply put: we first modify one copy and then the other. This ensures
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| 302 | * there is always one copy in a stable state, ready to give us an answer.
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| 303 | *
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| 304 | * The basic form is a data structure like:
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| 305 | *
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| 306 | * struct latch_struct {
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| 307 | * seqcount_t seq;
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| 308 | * struct data_struct data[2];
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| 309 | * };
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| 310 | *
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| 311 | * Where a modification, which is assumed to be externally serialized, does the
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| 312 | * following:
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| 313 | *
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| 314 | * void latch_modify(struct latch_struct *latch, ...)
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| 315 | * {
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| 316 | * smp_wmb(); <- Ensure that the last data[1] update is visible
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| 317 | * latch->seq++;
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| 318 | * smp_wmb(); <- Ensure that the seqcount update is visible
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| 319 | *
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| 320 | * modify(latch->data[0], ...);
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| 321 | *
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| 322 | * smp_wmb(); <- Ensure that the data[0] update is visible
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| 323 | * latch->seq++;
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| 324 | * smp_wmb(); <- Ensure that the seqcount update is visible
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| 325 | *
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| 326 | * modify(latch->data[1], ...);
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| 327 | * }
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| 328 | *
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| 329 | * The query will have a form like:
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| 330 | *
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| 331 | * struct entry *latch_query(struct latch_struct *latch, ...)
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| 332 | * {
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| 333 | * struct entry *entry;
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| 334 | * unsigned seq, idx;
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| 335 | *
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| 336 | * do {
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| 337 | * seq = raw_read_seqcount_latch(&latch->seq);
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| 338 | *
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| 339 | * idx = seq & 0x01;
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| 340 | * entry = data_query(latch->data[idx], ...);
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| 341 | *
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| 342 | * smp_rmb();
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| 343 | * } while (seq != latch->seq);
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| 344 | *
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| 345 | * return entry;
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| 346 | * }
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| 347 | *
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| 348 | * So during the modification, queries are first redirected to data[1]. Then we
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| 349 | * modify data[0]. When that is complete, we redirect queries back to data[0]
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| 350 | * and we can modify data[1].
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| 351 | *
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| 352 | * NOTE: The non-requirement for atomic modifications does _NOT_ include
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| 353 | * the publishing of new entries in the case where data is a dynamic
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| 354 | * data structure.
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| 355 | *
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| 356 | * An iteration might start in data[0] and get suspended long enough
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| 357 | * to miss an entire modification sequence, once it resumes it might
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| 358 | * observe the new entry.
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| 359 | *
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| 360 | * NOTE: When data is a dynamic data structure; one should use regular RCU
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| 361 | * patterns to manage the lifetimes of the objects within.
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| 362 |
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| 363 | static inline void raw_write_seqcount_latch(seqcount_t *s)
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| 364 | {
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| 365 | smp_wmb();
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| 366 | s->sequence++;
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| 367 | smp_wmb();
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| 368 | }
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| 369 |
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| 370 | |
| 371 | * Sequence counter only version assumes that callers are using their
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| 372 | * own mutexing.
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| 373 |
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| 374 | static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
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| 375 | {
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| 376 | raw_write_seqcount_begin(s);
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| 377 | seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
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| 378 | }
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| 379 |
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| 380 | static inline void write_seqcount_begin(seqcount_t *s)
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| 381 | {
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| 382 | write_seqcount_begin_nested(s, 0);
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| 383 | }
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| 384 |
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| 385 | static inline void write_seqcount_end(seqcount_t *s)
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| 386 | {
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| 387 | seqcount_release(&s->dep_map, 1, _RET_IP_);
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| 388 | raw_write_seqcount_end(s);
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| 389 | }
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| 390 |
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| 391 | |
| 392 | * write_seqcount_invalidate - invalidate in-progress read-side seq operations
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| 393 | * @s: pointer to seqcount_t
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| 394 | *
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| 395 | * After write_seqcount_invalidate, no read-side seq operations will complete
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| 396 | * successfully and see data older than this.
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| 397 |
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| 398 | static inline void write_seqcount_invalidate(seqcount_t *s)
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| 399 | {
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| 400 | smp_wmb();
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| 401 | s->sequence+=2;
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| 402 | }
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| 403 |
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| 404 | typedef struct {
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| 405 | struct seqcount seqcount;
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| 406 | spinlock_t lock;
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| 407 | } seqlock_t;
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| 408 |
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| 409 | |
| 410 | * These macros triggered gcc-3.x compile-time problems. We think these are
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| 411 | * OK now. Be cautious.
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| 412 |
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| 413 | #define __SEQLOCK_UNLOCKED(lockname) \
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| 414 | { \
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| 415 | .seqcount = SEQCNT_ZERO(lockname), \
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| 416 | .lock = __SPIN_LOCK_UNLOCKED(lockname) \
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| 417 | }
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| 418 |
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| 419 | #define seqlock_init(x) \
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| 420 | do { \
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| 421 | seqcount_init(&(x)->seqcount); \
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| 422 | spin_lock_init(&(x)->lock); \
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| 423 | } while (0)
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| 424 |
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| 425 | #define DEFINE_SEQLOCK(x) \
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| 426 | seqlock_t x = __SEQLOCK_UNLOCKED(x)
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| 427 |
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| 428 | |
| 429 | * Read side functions for starting and finalizing a read side section.
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| 430 |
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| 431 | static inline unsigned read_seqbegin(const seqlock_t *sl)
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| 432 | {
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| 433 | return read_seqcount_begin(&sl->seqcount);
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| 434 | }
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| 435 |
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| 436 | static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
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| 437 | {
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| 438 | return read_seqcount_retry(&sl->seqcount, start);
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| 439 | }
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| 440 |
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| 441 | |
| 442 | * Lock out other writers and update the count.
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| 443 | * Acts like a normal spin_lock/unlock.
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| 444 | * Don't need preempt_disable() because that is in the spin_lock already.
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| 445 |
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| 446 | static inline void write_seqlock(seqlock_t *sl)
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| 447 | {
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| 448 | spin_lock(&sl->lock);
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| 449 | write_seqcount_begin(&sl->seqcount);
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| 450 | }
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| 451 |
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| 452 | static inline void write_sequnlock(seqlock_t *sl)
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| 453 | {
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| 454 | write_seqcount_end(&sl->seqcount);
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| 455 | spin_unlock(&sl->lock);
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| 456 | }
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| 457 |
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| 458 | static inline void write_seqlock_bh(seqlock_t *sl)
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| 459 | {
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| 460 | spin_lock_bh(&sl->lock);
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| 461 | write_seqcount_begin(&sl->seqcount);
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| 462 | }
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| 463 |
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| 464 | static inline void write_sequnlock_bh(seqlock_t *sl)
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| 465 | {
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| 466 | write_seqcount_end(&sl->seqcount);
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| 467 | spin_unlock_bh(&sl->lock);
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| 468 | }
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| 469 |
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| 470 | static inline void write_seqlock_irq(seqlock_t *sl)
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| 471 | {
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| 472 | spin_lock_irq(&sl->lock);
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| 473 | write_seqcount_begin(&sl->seqcount);
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| 474 | }
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| 475 |
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| 476 | static inline void write_sequnlock_irq(seqlock_t *sl)
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| 477 | {
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| 478 | write_seqcount_end(&sl->seqcount);
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| 479 | spin_unlock_irq(&sl->lock);
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| 480 | }
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| 481 |
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| 482 | static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
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| 483 | {
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| 484 | unsigned long flags;
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| 485 |
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| 486 | spin_lock_irqsave(&sl->lock, flags);
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| 487 | write_seqcount_begin(&sl->seqcount);
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| 488 | return flags;
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| 489 | }
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| 490 |
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| 491 | #define write_seqlock_irqsave(lock, flags) \
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| 492 | do { flags = __write_seqlock_irqsave(lock); } while (0)
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| 493 |
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| 494 | static inline void
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| 495 | write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
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| 496 | {
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| 497 | write_seqcount_end(&sl->seqcount);
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| 498 | spin_unlock_irqrestore(&sl->lock, flags);
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| 499 | }
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| 500 |
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| 501 | |
| 502 | * A locking reader exclusively locks out other writers and locking readers,
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| 503 | * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
|
| 504 | * Don't need preempt_disable() because that is in the spin_lock already.
|
| 505 |
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| 506 | static inline void read_seqlock_excl(seqlock_t *sl)
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| 507 | {
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| 508 | spin_lock(&sl->lock);
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| 509 | }
|
| 510 |
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| 511 | static inline void read_sequnlock_excl(seqlock_t *sl)
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| 512 | {
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| 513 | spin_unlock(&sl->lock);
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| 514 | }
|
| 515 |
|
| 516 | |
| 517 | * read_seqbegin_or_lock - begin a sequence number check or locking block
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| 518 | * @lock: sequence lock
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| 519 | * @seq : sequence number to be checked
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| 520 | *
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| 521 | * First try it once optimistically without taking the lock. If that fails,
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| 522 | * take the lock. The sequence number is also used as a marker for deciding
|
| 523 | * whether to be a reader (even) or writer (odd).
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| 524 | * N.B. seq must be initialized to an even number to begin with.
|
| 525 |
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| 526 | static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
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| 527 | {
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| 528 | if (!(*seq & 1))
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| 529 | *seq = read_seqbegin(lock);
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| 530 | else
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| 531 | read_seqlock_excl(lock);
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| 532 | }
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| 533 |
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| 534 | static inline int need_seqretry(seqlock_t *lock, int seq)
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| 535 | {
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| 536 | return !(seq & 1) && read_seqretry(lock, seq);
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| 537 | }
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| 538 |
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| 539 | static inline void done_seqretry(seqlock_t *lock, int seq)
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| 540 | {
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| 541 | if (seq & 1)
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| 542 | read_sequnlock_excl(lock);
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| 543 | }
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| 544 |
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| 545 | static inline void read_seqlock_excl_bh(seqlock_t *sl)
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| 546 | {
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| 547 | spin_lock_bh(&sl->lock);
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| 548 | }
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| 549 |
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| 550 | static inline void read_sequnlock_excl_bh(seqlock_t *sl)
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| 551 | {
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| 552 | spin_unlock_bh(&sl->lock);
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| 553 | }
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| 554 |
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| 555 | static inline void read_seqlock_excl_irq(seqlock_t *sl)
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| 556 | {
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| 557 | spin_lock_irq(&sl->lock);
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| 558 | }
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| 559 |
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| 560 | static inline void read_sequnlock_excl_irq(seqlock_t *sl)
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| 561 | {
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| 562 | spin_unlock_irq(&sl->lock);
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| 563 | }
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| 564 |
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| 565 | static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
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| 566 | {
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| 567 | unsigned long flags;
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| 568 |
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| 569 | spin_lock_irqsave(&sl->lock, flags);
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| 570 | return flags;
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| 571 | }
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| 572 |
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| 573 | #define read_seqlock_excl_irqsave(lock, flags) \
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| 574 | do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
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| 575 |
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| 576 | static inline void
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| 577 | read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
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| 578 | {
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| 579 | spin_unlock_irqrestore(&sl->lock, flags);
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| 580 | }
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| 581 |
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| 582 | static inline unsigned long
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| 583 | read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
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| 584 | {
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| 585 | unsigned long flags = 0;
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| 586 |
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| 587 | if (!(*seq & 1))
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| 588 | *seq = read_seqbegin(lock);
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| 589 | else
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| 590 | read_seqlock_excl_irqsave(lock, flags);
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| 591 |
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| 592 | return flags;
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| 593 | }
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| 594 |
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| 595 | static inline void
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| 596 | done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
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| 597 | {
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| 598 | if (seq & 1)
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| 599 | read_sequnlock_excl_irqrestore(lock, flags);
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| 600 | }
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| 601 | #endif
|