1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef _GEN_PV_LOCK_SLOWPATH
3 #error "do not include this file"
4 #endif
5 
6 #include <linux/hash.h>
7 #include <linux/memblock.h>
8 #include <linux/debug_locks.h>
9 
10 /*
11  * Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
12  * of spinning them.
13  *
14  * This relies on the architecture to provide two paravirt hypercalls:
15  *
16  *   pv_wait(u8 *ptr, u8 val) -- suspends the vcpu if *ptr == val
17  *   pv_kick(cpu)             -- wakes a suspended vcpu
18  *
19  * Using these we implement __pv_queued_spin_lock_slowpath() and
20  * __pv_queued_spin_unlock() to replace native_queued_spin_lock_slowpath() and
21  * native_queued_spin_unlock().
22  */
23 
24 #define _Q_SLOW_VAL	(3U << _Q_LOCKED_OFFSET)
25 
26 /*
27  * Queue Node Adaptive Spinning
28  *
29  * A queue node vCPU will stop spinning if the vCPU in the previous node is
30  * not running. The one lock stealing attempt allowed at slowpath entry
31  * mitigates the slight slowdown for non-overcommitted guest with this
32  * aggressive wait-early mechanism.
33  *
34  * The status of the previous node will be checked at fixed interval
35  * controlled by PV_PREV_CHECK_MASK. This is to ensure that we won't
36  * pound on the cacheline of the previous node too heavily.
37  */
38 #define PV_PREV_CHECK_MASK	0xff
39 
40 /*
41  * Queue node uses: vcpu_running & vcpu_halted.
42  * Queue head uses: vcpu_running & vcpu_hashed.
43  */
44 enum vcpu_state {
45 	vcpu_running = 0,
46 	vcpu_halted,		/* Used only in pv_wait_node */
47 	vcpu_hashed,		/* = pv_hash'ed + vcpu_halted */
48 };
49 
50 struct pv_node {
51 	struct mcs_spinlock	mcs;
52 	int			cpu;
53 	u8			state;
54 };
55 
56 /*
57  * Hybrid PV queued/unfair lock
58  *
59  * By replacing the regular queued_spin_trylock() with the function below,
60  * it will be called once when a lock waiter enter the PV slowpath before
61  * being queued.
62  *
63  * The pending bit is set by the queue head vCPU of the MCS wait queue in
64  * pv_wait_head_or_lock() to signal that it is ready to spin on the lock.
65  * When that bit becomes visible to the incoming waiters, no lock stealing
66  * is allowed. The function will return immediately to make the waiters
67  * enter the MCS wait queue. So lock starvation shouldn't happen as long
68  * as the queued mode vCPUs are actively running to set the pending bit
69  * and hence disabling lock stealing.
70  *
71  * When the pending bit isn't set, the lock waiters will stay in the unfair
72  * mode spinning on the lock unless the MCS wait queue is empty. In this
73  * case, the lock waiters will enter the queued mode slowpath trying to
74  * become the queue head and set the pending bit.
75  *
76  * This hybrid PV queued/unfair lock combines the best attributes of a
77  * queued lock (no lock starvation) and an unfair lock (good performance
78  * on not heavily contended locks).
79  */
80 #define queued_spin_trylock(l)	pv_hybrid_queued_unfair_trylock(l)
81 static inline bool pv_hybrid_queued_unfair_trylock(struct qspinlock *lock)
82 {
83 	/*
84 	 * Stay in unfair lock mode as long as queued mode waiters are
85 	 * present in the MCS wait queue but the pending bit isn't set.
86 	 */
87 	for (;;) {
88 		int val = atomic_read(&lock->val);
89 
90 		if (!(val & _Q_LOCKED_PENDING_MASK) &&
91 		   (cmpxchg_acquire(&lock->locked, 0, _Q_LOCKED_VAL) == 0)) {
92 			lockevent_inc(pv_lock_stealing);
93 			return true;
94 		}
95 		if (!(val & _Q_TAIL_MASK) || (val & _Q_PENDING_MASK))
96 			break;
97 
98 		cpu_relax();
99 	}
100 
101 	return false;
102 }
103 
104 /*
105  * The pending bit is used by the queue head vCPU to indicate that it
106  * is actively spinning on the lock and no lock stealing is allowed.
107  */
108 #if _Q_PENDING_BITS == 8
109 static __always_inline void set_pending(struct qspinlock *lock)
110 {
111 	WRITE_ONCE(lock->pending, 1);
112 }
113 
114 /*
115  * The pending bit check in pv_queued_spin_steal_lock() isn't a memory
116  * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the
117  * lock just to be sure that it will get it.
118  */
119 static __always_inline int trylock_clear_pending(struct qspinlock *lock)
120 {
121 	return !READ_ONCE(lock->locked) &&
122 	       (cmpxchg_acquire(&lock->locked_pending, _Q_PENDING_VAL,
123 				_Q_LOCKED_VAL) == _Q_PENDING_VAL);
124 }
125 #else /* _Q_PENDING_BITS == 8 */
126 static __always_inline void set_pending(struct qspinlock *lock)
127 {
128 	atomic_or(_Q_PENDING_VAL, &lock->val);
129 }
130 
131 static __always_inline int trylock_clear_pending(struct qspinlock *lock)
132 {
133 	int val = atomic_read(&lock->val);
134 
135 	for (;;) {
136 		int old, new;
137 
138 		if (val  & _Q_LOCKED_MASK)
139 			break;
140 
141 		/*
142 		 * Try to clear pending bit & set locked bit
143 		 */
144 		old = val;
145 		new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
146 		val = atomic_cmpxchg_acquire(&lock->val, old, new);
147 
148 		if (val == old)
149 			return 1;
150 	}
151 	return 0;
152 }
153 #endif /* _Q_PENDING_BITS == 8 */
154 
155 /*
156  * Lock and MCS node addresses hash table for fast lookup
157  *
158  * Hashing is done on a per-cacheline basis to minimize the need to access
159  * more than one cacheline.
160  *
161  * Dynamically allocate a hash table big enough to hold at least 4X the
162  * number of possible cpus in the system. Allocation is done on page
163  * granularity. So the minimum number of hash buckets should be at least
164  * 256 (64-bit) or 512 (32-bit) to fully utilize a 4k page.
165  *
166  * Since we should not be holding locks from NMI context (very rare indeed) the
167  * max load factor is 0.75, which is around the point where open addressing
168  * breaks down.
169  *
170  */
171 struct pv_hash_entry {
172 	struct qspinlock *lock;
173 	struct pv_node   *node;
174 };
175 
176 #define PV_HE_PER_LINE	(SMP_CACHE_BYTES / sizeof(struct pv_hash_entry))
177 #define PV_HE_MIN	(PAGE_SIZE / sizeof(struct pv_hash_entry))
178 
179 static struct pv_hash_entry *pv_lock_hash;
180 static unsigned int pv_lock_hash_bits __read_mostly;
181 
182 /*
183  * Allocate memory for the PV qspinlock hash buckets
184  *
185  * This function should be called from the paravirt spinlock initialization
186  * routine.
187  */
188 void __init __pv_init_lock_hash(void)
189 {
190 	int pv_hash_size = ALIGN(4 * num_possible_cpus(), PV_HE_PER_LINE);
191 
192 	if (pv_hash_size < PV_HE_MIN)
193 		pv_hash_size = PV_HE_MIN;
194 
195 	/*
196 	 * Allocate space from bootmem which should be page-size aligned
197 	 * and hence cacheline aligned.
198 	 */
199 	pv_lock_hash = alloc_large_system_hash("PV qspinlock",
200 					       sizeof(struct pv_hash_entry),
201 					       pv_hash_size, 0,
202 					       HASH_EARLY | HASH_ZERO,
203 					       &pv_lock_hash_bits, NULL,
204 					       pv_hash_size, pv_hash_size);
205 }
206 
207 #define for_each_hash_entry(he, offset, hash)						\
208 	for (hash &= ~(PV_HE_PER_LINE - 1), he = &pv_lock_hash[hash], offset = 0;	\
209 	     offset < (1 << pv_lock_hash_bits);						\
210 	     offset++, he = &pv_lock_hash[(hash + offset) & ((1 << pv_lock_hash_bits) - 1)])
211 
212 static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node)
213 {
214 	unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
215 	struct pv_hash_entry *he;
216 	int hopcnt = 0;
217 
218 	for_each_hash_entry(he, offset, hash) {
219 		hopcnt++;
220 		if (!cmpxchg(&he->lock, NULL, lock)) {
221 			WRITE_ONCE(he->node, node);
222 			lockevent_pv_hop(hopcnt);
223 			return &he->lock;
224 		}
225 	}
226 	/*
227 	 * Hard assume there is a free entry for us.
228 	 *
229 	 * This is guaranteed by ensuring every blocked lock only ever consumes
230 	 * a single entry, and since we only have 4 nesting levels per CPU
231 	 * and allocated 4*nr_possible_cpus(), this must be so.
232 	 *
233 	 * The single entry is guaranteed by having the lock owner unhash
234 	 * before it releases.
235 	 */
236 	BUG();
237 }
238 
239 static struct pv_node *pv_unhash(struct qspinlock *lock)
240 {
241 	unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
242 	struct pv_hash_entry *he;
243 	struct pv_node *node;
244 
245 	for_each_hash_entry(he, offset, hash) {
246 		if (READ_ONCE(he->lock) == lock) {
247 			node = READ_ONCE(he->node);
248 			WRITE_ONCE(he->lock, NULL);
249 			return node;
250 		}
251 	}
252 	/*
253 	 * Hard assume we'll find an entry.
254 	 *
255 	 * This guarantees a limited lookup time and is itself guaranteed by
256 	 * having the lock owner do the unhash -- IFF the unlock sees the
257 	 * SLOW flag, there MUST be a hash entry.
258 	 */
259 	BUG();
260 }
261 
262 /*
263  * Return true if when it is time to check the previous node which is not
264  * in a running state.
265  */
266 static inline bool
267 pv_wait_early(struct pv_node *prev, int loop)
268 {
269 	if ((loop & PV_PREV_CHECK_MASK) != 0)
270 		return false;
271 
272 	return READ_ONCE(prev->state) != vcpu_running;
273 }
274 
275 /*
276  * Initialize the PV part of the mcs_spinlock node.
277  */
278 static void pv_init_node(struct mcs_spinlock *node)
279 {
280 	struct pv_node *pn = (struct pv_node *)node;
281 
282 	BUILD_BUG_ON(sizeof(struct pv_node) > sizeof(struct qnode));
283 
284 	pn->cpu = smp_processor_id();
285 	pn->state = vcpu_running;
286 }
287 
288 /*
289  * Wait for node->locked to become true, halt the vcpu after a short spin.
290  * pv_kick_node() is used to set _Q_SLOW_VAL and fill in hash table on its
291  * behalf.
292  */
293 static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
294 {
295 	struct pv_node *pn = (struct pv_node *)node;
296 	struct pv_node *pp = (struct pv_node *)prev;
297 	int loop;
298 	bool wait_early;
299 
300 	for (;;) {
301 		for (wait_early = false, loop = SPIN_THRESHOLD; loop; loop--) {
302 			if (READ_ONCE(node->locked))
303 				return;
304 			if (pv_wait_early(pp, loop)) {
305 				wait_early = true;
306 				break;
307 			}
308 			cpu_relax();
309 		}
310 
311 		/*
312 		 * Order pn->state vs pn->locked thusly:
313 		 *
314 		 * [S] pn->state = vcpu_halted	  [S] next->locked = 1
315 		 *     MB			      MB
316 		 * [L] pn->locked		[RmW] pn->state = vcpu_hashed
317 		 *
318 		 * Matches the cmpxchg() from pv_kick_node().
319 		 */
320 		smp_store_mb(pn->state, vcpu_halted);
321 
322 		if (!READ_ONCE(node->locked)) {
323 			lockevent_inc(pv_wait_node);
324 			lockevent_cond_inc(pv_wait_early, wait_early);
325 			pv_wait(&pn->state, vcpu_halted);
326 		}
327 
328 		/*
329 		 * If pv_kick_node() changed us to vcpu_hashed, retain that
330 		 * value so that pv_wait_head_or_lock() knows to not also try
331 		 * to hash this lock.
332 		 */
333 		cmpxchg(&pn->state, vcpu_halted, vcpu_running);
334 
335 		/*
336 		 * If the locked flag is still not set after wakeup, it is a
337 		 * spurious wakeup and the vCPU should wait again. However,
338 		 * there is a pretty high overhead for CPU halting and kicking.
339 		 * So it is better to spin for a while in the hope that the
340 		 * MCS lock will be released soon.
341 		 */
342 		lockevent_cond_inc(pv_spurious_wakeup,
343 				  !READ_ONCE(node->locked));
344 	}
345 
346 	/*
347 	 * By now our node->locked should be 1 and our caller will not actually
348 	 * spin-wait for it. We do however rely on our caller to do a
349 	 * load-acquire for us.
350 	 */
351 }
352 
353 /*
354  * Called after setting next->locked = 1 when we're the lock owner.
355  *
356  * Instead of waking the waiters stuck in pv_wait_node() advance their state
357  * such that they're waiting in pv_wait_head_or_lock(), this avoids a
358  * wake/sleep cycle.
359  */
360 static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
361 {
362 	struct pv_node *pn = (struct pv_node *)node;
363 
364 	/*
365 	 * If the vCPU is indeed halted, advance its state to match that of
366 	 * pv_wait_node(). If OTOH this fails, the vCPU was running and will
367 	 * observe its next->locked value and advance itself.
368 	 *
369 	 * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
370 	 *
371 	 * The write to next->locked in arch_mcs_spin_unlock_contended()
372 	 * must be ordered before the read of pn->state in the cmpxchg()
373 	 * below for the code to work correctly. To guarantee full ordering
374 	 * irrespective of the success or failure of the cmpxchg(),
375 	 * a relaxed version with explicit barrier is used. The control
376 	 * dependency will order the reading of pn->state before any
377 	 * subsequent writes.
378 	 */
379 	smp_mb__before_atomic();
380 	if (cmpxchg_relaxed(&pn->state, vcpu_halted, vcpu_hashed)
381 	    != vcpu_halted)
382 		return;
383 
384 	/*
385 	 * Put the lock into the hash table and set the _Q_SLOW_VAL.
386 	 *
387 	 * As this is the same vCPU that will check the _Q_SLOW_VAL value and
388 	 * the hash table later on at unlock time, no atomic instruction is
389 	 * needed.
390 	 */
391 	WRITE_ONCE(lock->locked, _Q_SLOW_VAL);
392 	(void)pv_hash(lock, pn);
393 }
394 
395 /*
396  * Wait for l->locked to become clear and acquire the lock;
397  * halt the vcpu after a short spin.
398  * __pv_queued_spin_unlock() will wake us.
399  *
400  * The current value of the lock will be returned for additional processing.
401  */
402 static u32
403 pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
404 {
405 	struct pv_node *pn = (struct pv_node *)node;
406 	struct qspinlock **lp = NULL;
407 	int waitcnt = 0;
408 	int loop;
409 
410 	/*
411 	 * If pv_kick_node() already advanced our state, we don't need to
412 	 * insert ourselves into the hash table anymore.
413 	 */
414 	if (READ_ONCE(pn->state) == vcpu_hashed)
415 		lp = (struct qspinlock **)1;
416 
417 	/*
418 	 * Tracking # of slowpath locking operations
419 	 */
420 	lockevent_inc(lock_slowpath);
421 
422 	for (;; waitcnt++) {
423 		/*
424 		 * Set correct vCPU state to be used by queue node wait-early
425 		 * mechanism.
426 		 */
427 		WRITE_ONCE(pn->state, vcpu_running);
428 
429 		/*
430 		 * Set the pending bit in the active lock spinning loop to
431 		 * disable lock stealing before attempting to acquire the lock.
432 		 */
433 		set_pending(lock);
434 		for (loop = SPIN_THRESHOLD; loop; loop--) {
435 			if (trylock_clear_pending(lock))
436 				goto gotlock;
437 			cpu_relax();
438 		}
439 		clear_pending(lock);
440 
441 
442 		if (!lp) { /* ONCE */
443 			lp = pv_hash(lock, pn);
444 
445 			/*
446 			 * We must hash before setting _Q_SLOW_VAL, such that
447 			 * when we observe _Q_SLOW_VAL in __pv_queued_spin_unlock()
448 			 * we'll be sure to be able to observe our hash entry.
449 			 *
450 			 *   [S] <hash>                 [Rmw] l->locked == _Q_SLOW_VAL
451 			 *       MB                           RMB
452 			 * [RmW] l->locked = _Q_SLOW_VAL  [L] <unhash>
453 			 *
454 			 * Matches the smp_rmb() in __pv_queued_spin_unlock().
455 			 */
456 			if (xchg(&lock->locked, _Q_SLOW_VAL) == 0) {
457 				/*
458 				 * The lock was free and now we own the lock.
459 				 * Change the lock value back to _Q_LOCKED_VAL
460 				 * and unhash the table.
461 				 */
462 				WRITE_ONCE(lock->locked, _Q_LOCKED_VAL);
463 				WRITE_ONCE(*lp, NULL);
464 				goto gotlock;
465 			}
466 		}
467 		WRITE_ONCE(pn->state, vcpu_hashed);
468 		lockevent_inc(pv_wait_head);
469 		lockevent_cond_inc(pv_wait_again, waitcnt);
470 		pv_wait(&lock->locked, _Q_SLOW_VAL);
471 
472 		/*
473 		 * Because of lock stealing, the queue head vCPU may not be
474 		 * able to acquire the lock before it has to wait again.
475 		 */
476 	}
477 
478 	/*
479 	 * The cmpxchg() or xchg() call before coming here provides the
480 	 * acquire semantics for locking. The dummy ORing of _Q_LOCKED_VAL
481 	 * here is to indicate to the compiler that the value will always
482 	 * be nozero to enable better code optimization.
483 	 */
484 gotlock:
485 	return (u32)(atomic_read(&lock->val) | _Q_LOCKED_VAL);
486 }
487 
488 /*
489  * PV versions of the unlock fastpath and slowpath functions to be used
490  * instead of queued_spin_unlock().
491  */
492 __visible __lockfunc void
493 __pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8 locked)
494 {
495 	struct pv_node *node;
496 
497 	if (unlikely(locked != _Q_SLOW_VAL)) {
498 		WARN(!debug_locks_silent,
499 		     "pvqspinlock: lock 0x%lx has corrupted value 0x%x!\n",
500 		     (unsigned long)lock, atomic_read(&lock->val));
501 		return;
502 	}
503 
504 	/*
505 	 * A failed cmpxchg doesn't provide any memory-ordering guarantees,
506 	 * so we need a barrier to order the read of the node data in
507 	 * pv_unhash *after* we've read the lock being _Q_SLOW_VAL.
508 	 *
509 	 * Matches the cmpxchg() in pv_wait_head_or_lock() setting _Q_SLOW_VAL.
510 	 */
511 	smp_rmb();
512 
513 	/*
514 	 * Since the above failed to release, this must be the SLOW path.
515 	 * Therefore start by looking up the blocked node and unhashing it.
516 	 */
517 	node = pv_unhash(lock);
518 
519 	/*
520 	 * Now that we have a reference to the (likely) blocked pv_node,
521 	 * release the lock.
522 	 */
523 	smp_store_release(&lock->locked, 0);
524 
525 	/*
526 	 * At this point the memory pointed at by lock can be freed/reused,
527 	 * however we can still use the pv_node to kick the CPU.
528 	 * The other vCPU may not really be halted, but kicking an active
529 	 * vCPU is harmless other than the additional latency in completing
530 	 * the unlock.
531 	 */
532 	lockevent_inc(pv_kick_unlock);
533 	pv_kick(node->cpu);
534 }
535 
536 /*
537  * Include the architecture specific callee-save thunk of the
538  * __pv_queued_spin_unlock(). This thunk is put together with
539  * __pv_queued_spin_unlock() to make the callee-save thunk and the real unlock
540  * function close to each other sharing consecutive instruction cachelines.
541  * Alternatively, architecture specific version of __pv_queued_spin_unlock()
542  * can be defined.
543  */
544 #include <asm/qspinlock_paravirt.h>
545 
546 #ifndef __pv_queued_spin_unlock
547 __visible __lockfunc void __pv_queued_spin_unlock(struct qspinlock *lock)
548 {
549 	u8 locked;
550 
551 	/*
552 	 * We must not unlock if SLOW, because in that case we must first
553 	 * unhash. Otherwise it would be possible to have multiple @lock
554 	 * entries, which would be BAD.
555 	 */
556 	locked = cmpxchg_release(&lock->locked, _Q_LOCKED_VAL, 0);
557 	if (likely(locked == _Q_LOCKED_VAL))
558 		return;
559 
560 	__pv_queued_spin_unlock_slowpath(lock, locked);
561 }
562 #endif /* __pv_queued_spin_unlock */
563