| 1 | |
| 2 | * lib/kernel_lock.c
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| 3 | *
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| 4 | * This is the traditional BKL - big kernel lock. Largely
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| 5 | * relegated to obsolescence, but used by various less
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| 6 | * important (or lazy) subsystems.
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| 7 |
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| 8 | #include <linux/smp_lock.h>
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| 9 | #include <linux/module.h>
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| 10 | #include <linux/kallsyms.h>
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| 11 | #include <linux/semaphore.h>
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| 12 |
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| 13 | |
| 14 | * The 'big kernel lock'
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| 15 | *
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| 16 | * This spinlock is taken and released recursively by lock_kernel()
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| 17 | * and unlock_kernel(). It is transparently dropped and reacquired
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| 18 | * over schedule(). It is used to protect legacy code that hasn't
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| 19 | * been migrated to a proper locking design yet.
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| 20 | *
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| 21 | * Don't use in new code.
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| 22 |
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| 23 | static __cacheline_aligned_in_smp DEFINE_SPINLOCK(kernel_flag);
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| 24 |
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| 25 |
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| 26 | |
| 27 | * Acquire/release the underlying lock from the scheduler.
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| 28 | *
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| 29 | * This is called with preemption disabled, and should
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| 30 | * return an error value if it cannot get the lock and
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| 31 | * TIF_NEED_RESCHED gets set.
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| 32 | *
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| 33 | * If it successfully gets the lock, it should increment
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| 34 | * the preemption count like any spinlock does.
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| 35 | *
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| 36 | * (This works on UP too - _raw_spin_trylock will never
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| 37 | * return false in that case)
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| 38 |
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| 39 | int __lockfunc __reacquire_kernel_lock(void)
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| 40 | {
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| 41 | while (!_raw_spin_trylock(&kernel_flag)) {
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| 42 | if (test_thread_flag(TIF_NEED_RESCHED))
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| 43 | return -EAGAIN;
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| 44 | cpu_relax();
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| 45 | }
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| 46 | preempt_disable();
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| 47 | return 0;
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| 48 | }
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| 49 |
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| 50 | void __lockfunc __release_kernel_lock(void)
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| 51 | {
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| 52 | _raw_spin_unlock(&kernel_flag);
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| 53 | preempt_enable_no_resched();
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| 54 | }
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| 55 |
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| 56 | |
| 57 | * These are the BKL spinlocks - we try to be polite about preemption.
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| 58 | * If SMP is not on (ie UP preemption), this all goes away because the
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| 59 | * _raw_spin_trylock() will always succeed.
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| 60 |
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| 61 | #ifdef CONFIG_PREEMPT
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| 62 | static inline void __lock_kernel(void)
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| 63 | {
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| 64 | preempt_disable();
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| 65 | if (unlikely(!_raw_spin_trylock(&kernel_flag))) {
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| 66 | |
| 67 | * If preemption was disabled even before this
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| 68 | * was called, there's nothing we can be polite
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| 69 | * about - just spin.
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| 70 |
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| 71 | if (preempt_count() > 1) {
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| 72 | _raw_spin_lock(&kernel_flag);
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| 73 | return;
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| 74 | }
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| 75 |
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| 76 | |
| 77 | * Otherwise, let's wait for the kernel lock
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| 78 | * with preemption enabled..
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| 79 |
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| 80 | do {
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| 81 | preempt_enable();
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| 82 | while (spin_is_locked(&kernel_flag))
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| 83 | cpu_relax();
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| 84 | preempt_disable();
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| 85 | } while (!_raw_spin_trylock(&kernel_flag));
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| 86 | }
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| 87 | }
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| 88 |
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| 89 | #else
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| 90 |
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| 91 | |
| 92 | * Non-preemption case - just get the spinlock
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| 93 |
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| 94 | static inline void __lock_kernel(void)
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| 95 | {
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| 96 | _raw_spin_lock(&kernel_flag);
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| 97 | }
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| 98 | #endif
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| 99 |
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| 100 | static inline void __unlock_kernel(void)
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| 101 | {
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| 102 | |
| 103 | * the BKL is not covered by lockdep, so we open-code the
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| 104 | * unlocking sequence (and thus avoid the dep-chain ops):
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| 105 |
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| 106 | _raw_spin_unlock(&kernel_flag);
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| 107 | preempt_enable();
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| 108 | }
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| 109 |
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| 110 | |
| 111 | * Getting the big kernel lock.
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| 112 | *
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| 113 | * This cannot happen asynchronously, so we only need to
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| 114 | * worry about other CPU's.
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| 115 |
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| 116 | void __lockfunc lock_kernel(void)
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| 117 | {
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| 118 | int depth = current->lock_depth+1;
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| 119 | if (likely(!depth))
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| 120 | __lock_kernel();
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| 121 | current->lock_depth = depth;
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| 122 | }
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| 123 |
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| 124 | void __lockfunc unlock_kernel(void)
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| 125 | {
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| 126 | BUG_ON(current->lock_depth < 0);
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| 127 | if (likely(--current->lock_depth < 0))
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| 128 | __unlock_kernel();
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| 129 | }
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| 130 |
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| 131 | EXPORT_SYMBOL(lock_kernel);
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| 132 | EXPORT_SYMBOL(unlock_kernel);
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| 133 |
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