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Linux/kernel/locking/qspinlock_paravirt.h

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  1 #ifndef _GEN_PV_LOCK_SLOWPATH
  2 #error "do not include this file"
  3 #endif
  4 
  5 #include <linux/hash.h>
  6 #include <linux/bootmem.h>
  7 #include <linux/debug_locks.h>
  8 
  9 /*
 10  * Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
 11  * of spinning them.
 12  *
 13  * This relies on the architecture to provide two paravirt hypercalls:
 14  *
 15  *   pv_wait(u8 *ptr, u8 val) -- suspends the vcpu if *ptr == val
 16  *   pv_kick(cpu)             -- wakes a suspended vcpu
 17  *
 18  * Using these we implement __pv_queued_spin_lock_slowpath() and
 19  * __pv_queued_spin_unlock() to replace native_queued_spin_lock_slowpath() and
 20  * native_queued_spin_unlock().
 21  */
 22 
 23 #define _Q_SLOW_VAL     (3U << _Q_LOCKED_OFFSET)
 24 
 25 /*
 26  * Queue node uses: vcpu_running & vcpu_halted.
 27  * Queue head uses: vcpu_running & vcpu_hashed.
 28  */
 29 enum vcpu_state {
 30         vcpu_running = 0,
 31         vcpu_halted,            /* Used only in pv_wait_node */
 32         vcpu_hashed,            /* = pv_hash'ed + vcpu_halted */
 33 };
 34 
 35 struct pv_node {
 36         struct mcs_spinlock     mcs;
 37         struct mcs_spinlock     __res[3];
 38 
 39         int                     cpu;
 40         u8                      state;
 41 };
 42 
 43 /*
 44  * Lock and MCS node addresses hash table for fast lookup
 45  *
 46  * Hashing is done on a per-cacheline basis to minimize the need to access
 47  * more than one cacheline.
 48  *
 49  * Dynamically allocate a hash table big enough to hold at least 4X the
 50  * number of possible cpus in the system. Allocation is done on page
 51  * granularity. So the minimum number of hash buckets should be at least
 52  * 256 (64-bit) or 512 (32-bit) to fully utilize a 4k page.
 53  *
 54  * Since we should not be holding locks from NMI context (very rare indeed) the
 55  * max load factor is 0.75, which is around the point where open addressing
 56  * breaks down.
 57  *
 58  */
 59 struct pv_hash_entry {
 60         struct qspinlock *lock;
 61         struct pv_node   *node;
 62 };
 63 
 64 #define PV_HE_PER_LINE  (SMP_CACHE_BYTES / sizeof(struct pv_hash_entry))
 65 #define PV_HE_MIN       (PAGE_SIZE / sizeof(struct pv_hash_entry))
 66 
 67 static struct pv_hash_entry *pv_lock_hash;
 68 static unsigned int pv_lock_hash_bits __read_mostly;
 69 
 70 /*
 71  * Allocate memory for the PV qspinlock hash buckets
 72  *
 73  * This function should be called from the paravirt spinlock initialization
 74  * routine.
 75  */
 76 void __init __pv_init_lock_hash(void)
 77 {
 78         int pv_hash_size = ALIGN(4 * num_possible_cpus(), PV_HE_PER_LINE);
 79 
 80         if (pv_hash_size < PV_HE_MIN)
 81                 pv_hash_size = PV_HE_MIN;
 82 
 83         /*
 84          * Allocate space from bootmem which should be page-size aligned
 85          * and hence cacheline aligned.
 86          */
 87         pv_lock_hash = alloc_large_system_hash("PV qspinlock",
 88                                                sizeof(struct pv_hash_entry),
 89                                                pv_hash_size, 0, HASH_EARLY,
 90                                                &pv_lock_hash_bits, NULL,
 91                                                pv_hash_size, pv_hash_size);
 92 }
 93 
 94 #define for_each_hash_entry(he, offset, hash)                                           \
 95         for (hash &= ~(PV_HE_PER_LINE - 1), he = &pv_lock_hash[hash], offset = 0;       \
 96              offset < (1 << pv_lock_hash_bits);                                         \
 97              offset++, he = &pv_lock_hash[(hash + offset) & ((1 << pv_lock_hash_bits) - 1)])
 98 
 99 static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node)
100 {
101         unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
102         struct pv_hash_entry *he;
103 
104         for_each_hash_entry(he, offset, hash) {
105                 if (!cmpxchg(&he->lock, NULL, lock)) {
106                         WRITE_ONCE(he->node, node);
107                         return &he->lock;
108                 }
109         }
110         /*
111          * Hard assume there is a free entry for us.
112          *
113          * This is guaranteed by ensuring every blocked lock only ever consumes
114          * a single entry, and since we only have 4 nesting levels per CPU
115          * and allocated 4*nr_possible_cpus(), this must be so.
116          *
117          * The single entry is guaranteed by having the lock owner unhash
118          * before it releases.
119          */
120         BUG();
121 }
122 
123 static struct pv_node *pv_unhash(struct qspinlock *lock)
124 {
125         unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
126         struct pv_hash_entry *he;
127         struct pv_node *node;
128 
129         for_each_hash_entry(he, offset, hash) {
130                 if (READ_ONCE(he->lock) == lock) {
131                         node = READ_ONCE(he->node);
132                         WRITE_ONCE(he->lock, NULL);
133                         return node;
134                 }
135         }
136         /*
137          * Hard assume we'll find an entry.
138          *
139          * This guarantees a limited lookup time and is itself guaranteed by
140          * having the lock owner do the unhash -- IFF the unlock sees the
141          * SLOW flag, there MUST be a hash entry.
142          */
143         BUG();
144 }
145 
146 /*
147  * Initialize the PV part of the mcs_spinlock node.
148  */
149 static void pv_init_node(struct mcs_spinlock *node)
150 {
151         struct pv_node *pn = (struct pv_node *)node;
152 
153         BUILD_BUG_ON(sizeof(struct pv_node) > 5*sizeof(struct mcs_spinlock));
154 
155         pn->cpu = smp_processor_id();
156         pn->state = vcpu_running;
157 }
158 
159 /*
160  * Wait for node->locked to become true, halt the vcpu after a short spin.
161  * pv_kick_node() is used to set _Q_SLOW_VAL and fill in hash table on its
162  * behalf.
163  */
164 static void pv_wait_node(struct mcs_spinlock *node)
165 {
166         struct pv_node *pn = (struct pv_node *)node;
167         int loop;
168 
169         for (;;) {
170                 for (loop = SPIN_THRESHOLD; loop; loop--) {
171                         if (READ_ONCE(node->locked))
172                                 return;
173                         cpu_relax();
174                 }
175 
176                 /*
177                  * Order pn->state vs pn->locked thusly:
178                  *
179                  * [S] pn->state = vcpu_halted    [S] next->locked = 1
180                  *     MB                             MB
181                  * [L] pn->locked               [RmW] pn->state = vcpu_hashed
182                  *
183                  * Matches the cmpxchg() from pv_kick_node().
184                  */
185                 smp_store_mb(pn->state, vcpu_halted);
186 
187                 if (!READ_ONCE(node->locked))
188                         pv_wait(&pn->state, vcpu_halted);
189 
190                 /*
191                  * If pv_kick_node() changed us to vcpu_hashed, retain that value
192                  * so that pv_wait_head() knows to not also try to hash this lock.
193                  */
194                 cmpxchg(&pn->state, vcpu_halted, vcpu_running);
195 
196                 /*
197                  * If the locked flag is still not set after wakeup, it is a
198                  * spurious wakeup and the vCPU should wait again. However,
199                  * there is a pretty high overhead for CPU halting and kicking.
200                  * So it is better to spin for a while in the hope that the
201                  * MCS lock will be released soon.
202                  */
203         }
204 
205         /*
206          * By now our node->locked should be 1 and our caller will not actually
207          * spin-wait for it. We do however rely on our caller to do a
208          * load-acquire for us.
209          */
210 }
211 
212 /*
213  * Called after setting next->locked = 1 when we're the lock owner.
214  *
215  * Instead of waking the waiters stuck in pv_wait_node() advance their state such
216  * that they're waiting in pv_wait_head(), this avoids a wake/sleep cycle.
217  */
218 static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
219 {
220         struct pv_node *pn = (struct pv_node *)node;
221         struct __qspinlock *l = (void *)lock;
222 
223         /*
224          * If the vCPU is indeed halted, advance its state to match that of
225          * pv_wait_node(). If OTOH this fails, the vCPU was running and will
226          * observe its next->locked value and advance itself.
227          *
228          * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
229          */
230         if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted)
231                 return;
232 
233         /*
234          * Put the lock into the hash table and set the _Q_SLOW_VAL.
235          *
236          * As this is the same vCPU that will check the _Q_SLOW_VAL value and
237          * the hash table later on at unlock time, no atomic instruction is
238          * needed.
239          */
240         WRITE_ONCE(l->locked, _Q_SLOW_VAL);
241         (void)pv_hash(lock, pn);
242 }
243 
244 /*
245  * Wait for l->locked to become clear; halt the vcpu after a short spin.
246  * __pv_queued_spin_unlock() will wake us.
247  */
248 static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
249 {
250         struct pv_node *pn = (struct pv_node *)node;
251         struct __qspinlock *l = (void *)lock;
252         struct qspinlock **lp = NULL;
253         int loop;
254 
255         /*
256          * If pv_kick_node() already advanced our state, we don't need to
257          * insert ourselves into the hash table anymore.
258          */
259         if (READ_ONCE(pn->state) == vcpu_hashed)
260                 lp = (struct qspinlock **)1;
261 
262         for (;;) {
263                 for (loop = SPIN_THRESHOLD; loop; loop--) {
264                         if (!READ_ONCE(l->locked))
265                                 return;
266                         cpu_relax();
267                 }
268 
269                 if (!lp) { /* ONCE */
270                         WRITE_ONCE(pn->state, vcpu_hashed);
271                         lp = pv_hash(lock, pn);
272 
273                         /*
274                          * We must hash before setting _Q_SLOW_VAL, such that
275                          * when we observe _Q_SLOW_VAL in __pv_queued_spin_unlock()
276                          * we'll be sure to be able to observe our hash entry.
277                          *
278                          *   [S] pn->state
279                          *   [S] <hash>                 [Rmw] l->locked == _Q_SLOW_VAL
280                          *       MB                           RMB
281                          * [RmW] l->locked = _Q_SLOW_VAL  [L] <unhash>
282                          *                                [L] pn->state
283                          *
284                          * Matches the smp_rmb() in __pv_queued_spin_unlock().
285                          */
286                         if (!cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL)) {
287                                 /*
288                                  * The lock is free and _Q_SLOW_VAL has never
289                                  * been set. Therefore we need to unhash before
290                                  * getting the lock.
291                                  */
292                                 WRITE_ONCE(*lp, NULL);
293                                 return;
294                         }
295                 }
296                 pv_wait(&l->locked, _Q_SLOW_VAL);
297 
298                 /*
299                  * The unlocker should have freed the lock before kicking the
300                  * CPU. So if the lock is still not free, it is a spurious
301                  * wakeup and so the vCPU should wait again after spinning for
302                  * a while.
303                  */
304         }
305 
306         /*
307          * Lock is unlocked now; the caller will acquire it without waiting.
308          * As with pv_wait_node() we rely on the caller to do a load-acquire
309          * for us.
310          */
311 }
312 
313 /*
314  * PV version of the unlock function to be used in stead of
315  * queued_spin_unlock().
316  */
317 __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
318 {
319         struct __qspinlock *l = (void *)lock;
320         struct pv_node *node;
321         u8 locked;
322 
323         /*
324          * We must not unlock if SLOW, because in that case we must first
325          * unhash. Otherwise it would be possible to have multiple @lock
326          * entries, which would be BAD.
327          */
328         locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
329         if (likely(locked == _Q_LOCKED_VAL))
330                 return;
331 
332         if (unlikely(locked != _Q_SLOW_VAL)) {
333                 WARN(!debug_locks_silent,
334                      "pvqspinlock: lock 0x%lx has corrupted value 0x%x!\n",
335                      (unsigned long)lock, atomic_read(&lock->val));
336                 return;
337         }
338 
339         /*
340          * A failed cmpxchg doesn't provide any memory-ordering guarantees,
341          * so we need a barrier to order the read of the node data in
342          * pv_unhash *after* we've read the lock being _Q_SLOW_VAL.
343          *
344          * Matches the cmpxchg() in pv_wait_head() setting _Q_SLOW_VAL.
345          */
346         smp_rmb();
347 
348         /*
349          * Since the above failed to release, this must be the SLOW path.
350          * Therefore start by looking up the blocked node and unhashing it.
351          */
352         node = pv_unhash(lock);
353 
354         /*
355          * Now that we have a reference to the (likely) blocked pv_node,
356          * release the lock.
357          */
358         smp_store_release(&l->locked, 0);
359 
360         /*
361          * At this point the memory pointed at by lock can be freed/reused,
362          * however we can still use the pv_node to kick the CPU.
363          * The other vCPU may not really be halted, but kicking an active
364          * vCPU is harmless other than the additional latency in completing
365          * the unlock.
366          */
367         if (READ_ONCE(node->state) == vcpu_hashed)
368                 pv_kick(node->cpu);
369 }
370 /*
371  * Include the architecture specific callee-save thunk of the
372  * __pv_queued_spin_unlock(). This thunk is put together with
373  * __pv_queued_spin_unlock() near the top of the file to make sure
374  * that the callee-save thunk and the real unlock function are close
375  * to each other sharing consecutive instruction cachelines.
376  */
377 #include <asm/qspinlock_paravirt.h>
378 
379 

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