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1/*
2 * Queued spinlock
3 *
4 * This program is free software; you can redistribute it and/or modify
5 * it under the terms of the GNU General Public License as published by
6 * the Free Software Foundation; either version 2 of the License, or
7 * (at your option) any later version.
8 *
9 * This program is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 * GNU General Public License for more details.
13 *
14 * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
15 * (C) Copyright 2013-2014 Red Hat, Inc.
16 * (C) Copyright 2015 Intel Corp.
17 * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
18 *
19 * Authors: Waiman Long <[email protected]>
20 * Peter Zijlstra <[email protected]>
21 */
22
23#ifndef _GEN_PV_LOCK_SLOWPATH
24
25#include <linux/smp.h>
26#include <linux/bug.h>
27#include <linux/cpumask.h>
28#include <linux/percpu.h>
29#include <linux/hardirq.h>
30#include <linux/mutex.h>
31#include <asm/byteorder.h>
32#include <asm/qspinlock.h>
33
34/*
35 * The basic principle of a queue-based spinlock can best be understood
36 * by studying a classic queue-based spinlock implementation called the
37 * MCS lock. The paper below provides a good description for this kind
38 * of lock.
39 *
40 * http://www.cise.ufl.edu/tr/DOC/REP-1992-71.pdf
41 *
42 * This queued spinlock implementation is based on the MCS lock, however to make
43 * it fit the 4 bytes we assume spinlock_t to be, and preserve its existing
44 * API, we must modify it somehow.
45 *
46 * In particular; where the traditional MCS lock consists of a tail pointer
47 * (8 bytes) and needs the next pointer (another 8 bytes) of its own node to
48 * unlock the next pending (next->locked), we compress both these: {tail,
49 * next->locked} into a single u32 value.
50 *
51 * Since a spinlock disables recursion of its own context and there is a limit
52 * to the contexts that can nest; namely: task, softirq, hardirq, nmi. As there
53 * are at most 4 nesting levels, it can be encoded by a 2-bit number. Now
54 * we can encode the tail by combining the 2-bit nesting level with the cpu
55 * number. With one byte for the lock value and 3 bytes for the tail, only a
56 * 32-bit word is now needed. Even though we only need 1 bit for the lock,
57 * we extend it to a full byte to achieve better performance for architectures
58 * that support atomic byte write.
59 *
60 * We also change the first spinner to spin on the lock bit instead of its
61 * node; whereby avoiding the need to carry a node from lock to unlock, and
62 * preserving existing lock API. This also makes the unlock code simpler and
63 * faster.
64 *
65 * N.B. The current implementation only supports architectures that allow
66 * atomic operations on smaller 8-bit and 16-bit data types.
67 *
68 */
69
70#include "mcs_spinlock.h"
71
72#ifdef CONFIG_PARAVIRT_SPINLOCKS
73#define MAX_NODES 8
74#else
75#define MAX_NODES 4
76#endif
77
78/*
79 * Per-CPU queue node structures; we can never have more than 4 nested
80 * contexts: task, softirq, hardirq, nmi.
81 *
82 * Exactly fits one 64-byte cacheline on a 64-bit architecture.
83 *
84 * PV doubles the storage and uses the second cacheline for PV state.
85 */
86static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[MAX_NODES]);
87
88/*
89 * We must be able to distinguish between no-tail and the tail at 0:0,
90 * therefore increment the cpu number by one.
91 */
92
93static inline __pure u32 encode_tail(int cpu, int idx)
94{
95 u32 tail;
96
97#ifdef CONFIG_DEBUG_SPINLOCK
98 BUG_ON(idx > 3);
99#endif
100 tail = (cpu + 1) << _Q_TAIL_CPU_OFFSET;
101 tail |= idx << _Q_TAIL_IDX_OFFSET; /* assume < 4 */
102
103 return tail;
104}
105
106static inline __pure struct mcs_spinlock *decode_tail(u32 tail)
107{
108 int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1;
109 int idx = (tail & _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET;
110
111 return per_cpu_ptr(&mcs_nodes[idx], cpu);
112}
113
114#define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
115
116/*
117 * By using the whole 2nd least significant byte for the pending bit, we
118 * can allow better optimization of the lock acquisition for the pending
119 * bit holder.
120 *
121 * This internal structure is also used by the set_locked function which
122 * is not restricted to _Q_PENDING_BITS == 8.
123 */
124struct __qspinlock {
125 union {
126 atomic_t val;
127#ifdef __LITTLE_ENDIAN
128 struct {
129 u8 locked;
130 u8 pending;
131 };
132 struct {
133 u16 locked_pending;
134 u16 tail;
135 };
136#else
137 struct {
138 u16 tail;
139 u16 locked_pending;
140 };
141 struct {
142 u8 reserved[2];
143 u8 pending;
144 u8 locked;
145 };
146#endif
147 };
148};
149
150#if _Q_PENDING_BITS == 8
151/**
152 * clear_pending_set_locked - take ownership and clear the pending bit.
153 * @lock: Pointer to queued spinlock structure
154 *
155 * *,1,0 -> *,0,1
156 *
157 * Lock stealing is not allowed if this function is used.
158 */
159static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
160{
161 struct __qspinlock *l = (void *)lock;
162
163 WRITE_ONCE(l->locked_pending, _Q_LOCKED_VAL);
164}
165
166/*
167 * xchg_tail - Put in the new queue tail code word & retrieve previous one
168 * @lock : Pointer to queued spinlock structure
169 * @tail : The new queue tail code word
170 * Return: The previous queue tail code word
171 *
172 * xchg(lock, tail)
173 *
174 * p,*,* -> n,*,* ; prev = xchg(lock, node)
175 */
176static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
177{
178 struct __qspinlock *l = (void *)lock;
179
180 /*
181 * Use release semantics to make sure that the MCS node is properly
182 * initialized before changing the tail code.
183 */
184 return (u32)xchg_release(&l->tail,
185 tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
186}
187
188#else /* _Q_PENDING_BITS == 8 */
189
190/**
191 * clear_pending_set_locked - take ownership and clear the pending bit.
192 * @lock: Pointer to queued spinlock structure
193 *
194 * *,1,0 -> *,0,1
195 */
196static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
197{
198 atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);
199}
200
201/**
202 * xchg_tail - Put in the new queue tail code word & retrieve previous one
203 * @lock : Pointer to queued spinlock structure
204 * @tail : The new queue tail code word
205 * Return: The previous queue tail code word
206 *
207 * xchg(lock, tail)
208 *
209 * p,*,* -> n,*,* ; prev = xchg(lock, node)
210 */
211static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
212{
213 u32 old, new, val = atomic_read(&lock->val);
214
215 for (;;) {
216 new = (val & _Q_LOCKED_PENDING_MASK) | tail;
217 /*
218 * Use release semantics to make sure that the MCS node is
219 * properly initialized before changing the tail code.
220 */
221 old = atomic_cmpxchg_release(&lock->val, val, new);
222 if (old == val)
223 break;
224
225 val = old;
226 }
227 return old;
228}
229#endif /* _Q_PENDING_BITS == 8 */
230
231/**
232 * set_locked - Set the lock bit and own the lock
233 * @lock: Pointer to queued spinlock structure
234 *
235 * *,*,0 -> *,0,1
236 */
237static __always_inline void set_locked(struct qspinlock *lock)
238{
239 struct __qspinlock *l = (void *)lock;
240
241 WRITE_ONCE(l->locked, _Q_LOCKED_VAL);
242}
243
244
245/*
246 * Generate the native code for queued_spin_unlock_slowpath(); provide NOPs for
247 * all the PV callbacks.
248 */
249
250static __always_inline void __pv_init_node(struct mcs_spinlock *node) { }
251static __always_inline void __pv_wait_node(struct mcs_spinlock *node,
252 struct mcs_spinlock *prev) { }
253static __always_inline void __pv_kick_node(struct qspinlock *lock,
254 struct mcs_spinlock *node) { }
255static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,
256 struct mcs_spinlock *node)
257 { return 0; }
258
259#define pv_enabled() false
260
261#define pv_init_node __pv_init_node
262#define pv_wait_node __pv_wait_node
263#define pv_kick_node __pv_kick_node
264#define pv_wait_head_or_lock __pv_wait_head_or_lock
265
266#ifdef CONFIG_PARAVIRT_SPINLOCKS
267#define queued_spin_lock_slowpath native_queued_spin_lock_slowpath
268#endif
269
270/*
271 * Various notes on spin_is_locked() and spin_unlock_wait(), which are
272 * 'interesting' functions:
273 *
274 * PROBLEM: some architectures have an interesting issue with atomic ACQUIRE
275 * operations in that the ACQUIRE applies to the LOAD _not_ the STORE (ARM64,
276 * PPC). Also qspinlock has a similar issue per construction, the setting of
277 * the locked byte can be unordered acquiring the lock proper.
278 *
279 * This gets to be 'interesting' in the following cases, where the /should/s
280 * end up false because of this issue.
281 *
282 *
283 * CASE 1:
284 *
285 * So the spin_is_locked() correctness issue comes from something like:
286 *
287 * CPU0 CPU1
288 *
289 * global_lock(); local_lock(i)
290 * spin_lock(&G) spin_lock(&L[i])
291 * for (i) if (!spin_is_locked(&G)) {
292 * spin_unlock_wait(&L[i]); smp_acquire__after_ctrl_dep();
293 * return;
294 * }
295 * // deal with fail
296 *
297 * Where it is important CPU1 sees G locked or CPU0 sees L[i] locked such
298 * that there is exclusion between the two critical sections.
299 *
300 * The load from spin_is_locked(&G) /should/ be constrained by the ACQUIRE from
301 * spin_lock(&L[i]), and similarly the load(s) from spin_unlock_wait(&L[i])
302 * /should/ be constrained by the ACQUIRE from spin_lock(&G).
303 *
304 * Similarly, later stuff is constrained by the ACQUIRE from CTRL+RMB.
305 *
306 *
307 * CASE 2:
308 *
309 * For spin_unlock_wait() there is a second correctness issue, namely:
310 *
311 * CPU0 CPU1
312 *
313 * flag = set;
314 * smp_mb(); spin_lock(&l)
315 * spin_unlock_wait(&l); if (!flag)
316 * // add to lockless list
317 * spin_unlock(&l);
318 * // iterate lockless list
319 *
320 * Which wants to ensure that CPU1 will stop adding bits to the list and CPU0
321 * will observe the last entry on the list (if spin_unlock_wait() had ACQUIRE
322 * semantics etc..)
323 *
324 * Where flag /should/ be ordered against the locked store of l.
325 */
326
327/*
328 * queued_spin_lock_slowpath() can (load-)ACQUIRE the lock before
329 * issuing an _unordered_ store to set _Q_LOCKED_VAL.
330 *
331 * This means that the store can be delayed, but no later than the
332 * store-release from the unlock. This means that simply observing
333 * _Q_LOCKED_VAL is not sufficient to determine if the lock is acquired.
334 *
335 * There are two paths that can issue the unordered store:
336 *
337 * (1) clear_pending_set_locked(): *,1,0 -> *,0,1
338 *
339 * (2) set_locked(): t,0,0 -> t,0,1 ; t != 0
340 * atomic_cmpxchg_relaxed(): t,0,0 -> 0,0,1
341 *
342 * However, in both cases we have other !0 state we've set before to queue
343 * ourseves:
344 *
345 * For (1) we have the atomic_cmpxchg_acquire() that set _Q_PENDING_VAL, our
346 * load is constrained by that ACQUIRE to not pass before that, and thus must
347 * observe the store.
348 *
349 * For (2) we have a more intersting scenario. We enqueue ourselves using
350 * xchg_tail(), which ends up being a RELEASE. This in itself is not
351 * sufficient, however that is followed by an smp_cond_acquire() on the same
352 * word, giving a RELEASE->ACQUIRE ordering. This again constrains our load and
353 * guarantees we must observe that store.
354 *
355 * Therefore both cases have other !0 state that is observable before the
356 * unordered locked byte store comes through. This means we can use that to
357 * wait for the lock store, and then wait for an unlock.
358 */
359#ifndef queued_spin_unlock_wait
360void queued_spin_unlock_wait(struct qspinlock *lock)
361{
362 u32 val;
363
364 for (;;) {
365 val = atomic_read(&lock->val);
366
367 if (!val) /* not locked, we're done */
368 goto done;
369
370 if (val & _Q_LOCKED_MASK) /* locked, go wait for unlock */
371 break;
372
373 /* not locked, but pending, wait until we observe the lock */
374 cpu_relax();
375 }
376
377 /* any unlock is good */
378 while (atomic_read(&lock->val) & _Q_LOCKED_MASK)
379 cpu_relax();
380
381done:
382 smp_acquire__after_ctrl_dep();
383}
384EXPORT_SYMBOL(queued_spin_unlock_wait);
385#endif
386
387#endif /* _GEN_PV_LOCK_SLOWPATH */
388
389/**
390 * queued_spin_lock_slowpath - acquire the queued spinlock
391 * @lock: Pointer to queued spinlock structure
392 * @val: Current value of the queued spinlock 32-bit word
393 *
394 * (queue tail, pending bit, lock value)
395 *
396 * fast : slow : unlock
397 * : :
398 * uncontended (0,0,0) -:--> (0,0,1) ------------------------------:--> (*,*,0)
399 * : | ^--------.------. / :
400 * : v \ \ | :
401 * pending : (0,1,1) +--> (0,1,0) \ | :
402 * : | ^--' | | :
403 * : v | | :
404 * uncontended : (n,x,y) +--> (n,0,0) --' | :
405 * queue : | ^--' | :
406 * : v | :
407 * contended : (*,x,y) +--> (*,0,0) ---> (*,0,1) -' :
408 * queue : ^--' :
409 */
410void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
411{
412 struct mcs_spinlock *prev, *next, *node;
413 u32 new, old, tail;
414 int idx;
415
416 BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
417
418 if (pv_enabled())
419 goto queue;
420
421 if (virt_spin_lock(lock))
422 return;
423
424 /*
425 * wait for in-progress pending->locked hand-overs
426 *
427 * 0,1,0 -> 0,0,1
428 */
429 if (val == _Q_PENDING_VAL) {
430 while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)
431 cpu_relax();
432 }
433
434 /*
435 * trylock || pending
436 *
437 * 0,0,0 -> 0,0,1 ; trylock
438 * 0,0,1 -> 0,1,1 ; pending
439 */
440 for (;;) {
441 /*
442 * If we observe any contention; queue.
443 */
444 if (val & ~_Q_LOCKED_MASK)
445 goto queue;
446
447 new = _Q_LOCKED_VAL;
448 if (val == new)
449 new |= _Q_PENDING_VAL;
450
451 /*
452 * Acquire semantic is required here as the function may
453 * return immediately if the lock was free.
454 */
455 old = atomic_cmpxchg_acquire(&lock->val, val, new);
456 if (old == val)
457 break;
458
459 val = old;
460 }
461
462 /*
463 * we won the trylock
464 */
465 if (new == _Q_LOCKED_VAL)
466 return;
467
468 /*
469 * we're pending, wait for the owner to go away.
470 *
471 * *,1,1 -> *,1,0
472 *
473 * this wait loop must be a load-acquire such that we match the
474 * store-release that clears the locked bit and create lock
475 * sequentiality; this is because not all clear_pending_set_locked()
476 * implementations imply full barriers.
477 */
478 smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK));
479
480 /*
481 * take ownership and clear the pending bit.
482 *
483 * *,1,0 -> *,0,1
484 */
485 clear_pending_set_locked(lock);
486 return;
487
488 /*
489 * End of pending bit optimistic spinning and beginning of MCS
490 * queuing.
491 */
492queue:
493 node = this_cpu_ptr(&mcs_nodes[0]);
494 idx = node->count++;
495 tail = encode_tail(smp_processor_id(), idx);
496
497 node += idx;
498 node->locked = 0;
499 node->next = NULL;
500 pv_init_node(node);
501
502 /*
503 * We touched a (possibly) cold cacheline in the per-cpu queue node;
504 * attempt the trylock once more in the hope someone let go while we
505 * weren't watching.
506 */
507 if (queued_spin_trylock(lock))
508 goto release;
509
510 /*
511 * We have already touched the queueing cacheline; don't bother with
512 * pending stuff.
513 *
514 * p,*,* -> n,*,*
515 *
516 * RELEASE, such that the stores to @node must be complete.
517 */
518 old = xchg_tail(lock, tail);
519 next = NULL;
520
521 /*
522 * if there was a previous node; link it and wait until reaching the
523 * head of the waitqueue.
524 */
525 if (old & _Q_TAIL_MASK) {
526 prev = decode_tail(old);
527 /*
528 * The above xchg_tail() is also a load of @lock which generates,
529 * through decode_tail(), a pointer.
530 *
531 * The address dependency matches the RELEASE of xchg_tail()
532 * such that the access to @prev must happen after.
533 */
534 smp_read_barrier_depends();
535
536 WRITE_ONCE(prev->next, node);
537
538 pv_wait_node(node, prev);
539 arch_mcs_spin_lock_contended(&node->locked);
540
541 /*
542 * While waiting for the MCS lock, the next pointer may have
543 * been set by another lock waiter. We optimistically load
544 * the next pointer & prefetch the cacheline for writing
545 * to reduce latency in the upcoming MCS unlock operation.
546 */
547 next = READ_ONCE(node->next);
548 if (next)
549 prefetchw(next);
550 }
551
552 /*
553 * we're at the head of the waitqueue, wait for the owner & pending to
554 * go away.
555 *
556 * *,x,y -> *,0,0
557 *
558 * this wait loop must use a load-acquire such that we match the
559 * store-release that clears the locked bit and create lock
560 * sequentiality; this is because the set_locked() function below
561 * does not imply a full barrier.
562 *
563 * The PV pv_wait_head_or_lock function, if active, will acquire
564 * the lock and return a non-zero value. So we have to skip the
565 * smp_cond_load_acquire() call. As the next PV queue head hasn't been
566 * designated yet, there is no way for the locked value to become
567 * _Q_SLOW_VAL. So both the set_locked() and the
568 * atomic_cmpxchg_relaxed() calls will be safe.
569 *
570 * If PV isn't active, 0 will be returned instead.
571 *
572 */
573 if ((val = pv_wait_head_or_lock(lock, node)))
574 goto locked;
575
576 val = smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_PENDING_MASK));
577
578locked:
579 /*
580 * claim the lock:
581 *
582 * n,0,0 -> 0,0,1 : lock, uncontended
583 * *,0,0 -> *,0,1 : lock, contended
584 *
585 * If the queue head is the only one in the queue (lock value == tail),
586 * clear the tail code and grab the lock. Otherwise, we only need
587 * to grab the lock.
588 */
589 for (;;) {
590 /* In the PV case we might already have _Q_LOCKED_VAL set */
591 if ((val & _Q_TAIL_MASK) != tail) {
592 set_locked(lock);
593 break;
594 }
595 /*
596 * The smp_cond_load_acquire() call above has provided the
597 * necessary acquire semantics required for locking. At most
598 * two iterations of this loop may be ran.
599 */
600 old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
601 if (old == val)
602 goto release; /* No contention */
603
604 val = old;
605 }
606
607 /*
608 * contended path; wait for next if not observed yet, release.
609 */
610 if (!next) {
611 while (!(next = READ_ONCE(node->next)))
612 cpu_relax();
613 }
614
615 arch_mcs_spin_unlock_contended(&next->locked);
616 pv_kick_node(lock, next);
617
618release:
619 /*
620 * release the node
621 */
622 __this_cpu_dec(mcs_nodes[0].count);
623}
624EXPORT_SYMBOL(queued_spin_lock_slowpath);
625
626/*
627 * Generate the paravirt code for queued_spin_unlock_slowpath().
628 */
629#if !defined(_GEN_PV_LOCK_SLOWPATH) && defined(CONFIG_PARAVIRT_SPINLOCKS)
630#define _GEN_PV_LOCK_SLOWPATH
631
632#undef pv_enabled
633#define pv_enabled() true
634
635#undef pv_init_node
636#undef pv_wait_node
637#undef pv_kick_node
638#undef pv_wait_head_or_lock
639
640#undef queued_spin_lock_slowpath
641#define queued_spin_lock_slowpath __pv_queued_spin_lock_slowpath
642
643#include "qspinlock_paravirt.h"
644#include "qspinlock.c"
645
646#endif