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a23db284 WL |
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> | |
cba77f03 | 7 | #include <linux/debug_locks.h> |
a23db284 WL |
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 | enum vcpu_state { | |
26 | vcpu_running = 0, | |
27 | vcpu_halted, | |
28 | }; | |
29 | ||
30 | struct pv_node { | |
31 | struct mcs_spinlock mcs; | |
32 | struct mcs_spinlock __res[3]; | |
33 | ||
34 | int cpu; | |
35 | u8 state; | |
36 | }; | |
37 | ||
38 | /* | |
39 | * Lock and MCS node addresses hash table for fast lookup | |
40 | * | |
41 | * Hashing is done on a per-cacheline basis to minimize the need to access | |
42 | * more than one cacheline. | |
43 | * | |
44 | * Dynamically allocate a hash table big enough to hold at least 4X the | |
45 | * number of possible cpus in the system. Allocation is done on page | |
46 | * granularity. So the minimum number of hash buckets should be at least | |
47 | * 256 (64-bit) or 512 (32-bit) to fully utilize a 4k page. | |
48 | * | |
49 | * Since we should not be holding locks from NMI context (very rare indeed) the | |
50 | * max load factor is 0.75, which is around the point where open addressing | |
51 | * breaks down. | |
52 | * | |
53 | */ | |
54 | struct pv_hash_entry { | |
55 | struct qspinlock *lock; | |
56 | struct pv_node *node; | |
57 | }; | |
58 | ||
59 | #define PV_HE_PER_LINE (SMP_CACHE_BYTES / sizeof(struct pv_hash_entry)) | |
60 | #define PV_HE_MIN (PAGE_SIZE / sizeof(struct pv_hash_entry)) | |
61 | ||
62 | static struct pv_hash_entry *pv_lock_hash; | |
63 | static unsigned int pv_lock_hash_bits __read_mostly; | |
64 | ||
65 | /* | |
66 | * Allocate memory for the PV qspinlock hash buckets | |
67 | * | |
68 | * This function should be called from the paravirt spinlock initialization | |
69 | * routine. | |
70 | */ | |
71 | void __init __pv_init_lock_hash(void) | |
72 | { | |
73 | int pv_hash_size = ALIGN(4 * num_possible_cpus(), PV_HE_PER_LINE); | |
74 | ||
75 | if (pv_hash_size < PV_HE_MIN) | |
76 | pv_hash_size = PV_HE_MIN; | |
77 | ||
78 | /* | |
79 | * Allocate space from bootmem which should be page-size aligned | |
80 | * and hence cacheline aligned. | |
81 | */ | |
82 | pv_lock_hash = alloc_large_system_hash("PV qspinlock", | |
83 | sizeof(struct pv_hash_entry), | |
84 | pv_hash_size, 0, HASH_EARLY, | |
85 | &pv_lock_hash_bits, NULL, | |
86 | pv_hash_size, pv_hash_size); | |
87 | } | |
88 | ||
89 | #define for_each_hash_entry(he, offset, hash) \ | |
90 | for (hash &= ~(PV_HE_PER_LINE - 1), he = &pv_lock_hash[hash], offset = 0; \ | |
91 | offset < (1 << pv_lock_hash_bits); \ | |
92 | offset++, he = &pv_lock_hash[(hash + offset) & ((1 << pv_lock_hash_bits) - 1)]) | |
93 | ||
94 | static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node) | |
95 | { | |
96 | unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits); | |
97 | struct pv_hash_entry *he; | |
98 | ||
99 | for_each_hash_entry(he, offset, hash) { | |
100 | if (!cmpxchg(&he->lock, NULL, lock)) { | |
101 | WRITE_ONCE(he->node, node); | |
102 | return &he->lock; | |
103 | } | |
104 | } | |
105 | /* | |
106 | * Hard assume there is a free entry for us. | |
107 | * | |
108 | * This is guaranteed by ensuring every blocked lock only ever consumes | |
109 | * a single entry, and since we only have 4 nesting levels per CPU | |
110 | * and allocated 4*nr_possible_cpus(), this must be so. | |
111 | * | |
112 | * The single entry is guaranteed by having the lock owner unhash | |
113 | * before it releases. | |
114 | */ | |
115 | BUG(); | |
116 | } | |
117 | ||
118 | static struct pv_node *pv_unhash(struct qspinlock *lock) | |
119 | { | |
120 | unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits); | |
121 | struct pv_hash_entry *he; | |
122 | struct pv_node *node; | |
123 | ||
124 | for_each_hash_entry(he, offset, hash) { | |
125 | if (READ_ONCE(he->lock) == lock) { | |
126 | node = READ_ONCE(he->node); | |
127 | WRITE_ONCE(he->lock, NULL); | |
128 | return node; | |
129 | } | |
130 | } | |
131 | /* | |
132 | * Hard assume we'll find an entry. | |
133 | * | |
134 | * This guarantees a limited lookup time and is itself guaranteed by | |
135 | * having the lock owner do the unhash -- IFF the unlock sees the | |
136 | * SLOW flag, there MUST be a hash entry. | |
137 | */ | |
138 | BUG(); | |
139 | } | |
140 | ||
141 | /* | |
142 | * Initialize the PV part of the mcs_spinlock node. | |
143 | */ | |
144 | static void pv_init_node(struct mcs_spinlock *node) | |
145 | { | |
146 | struct pv_node *pn = (struct pv_node *)node; | |
147 | ||
148 | BUILD_BUG_ON(sizeof(struct pv_node) > 5*sizeof(struct mcs_spinlock)); | |
149 | ||
150 | pn->cpu = smp_processor_id(); | |
151 | pn->state = vcpu_running; | |
152 | } | |
153 | ||
154 | /* | |
155 | * Wait for node->locked to become true, halt the vcpu after a short spin. | |
156 | * pv_kick_node() is used to wake the vcpu again. | |
157 | */ | |
158 | static void pv_wait_node(struct mcs_spinlock *node) | |
159 | { | |
160 | struct pv_node *pn = (struct pv_node *)node; | |
161 | int loop; | |
162 | ||
163 | for (;;) { | |
164 | for (loop = SPIN_THRESHOLD; loop; loop--) { | |
165 | if (READ_ONCE(node->locked)) | |
166 | return; | |
167 | cpu_relax(); | |
168 | } | |
169 | ||
170 | /* | |
171 | * Order pn->state vs pn->locked thusly: | |
172 | * | |
173 | * [S] pn->state = vcpu_halted [S] next->locked = 1 | |
174 | * MB MB | |
175 | * [L] pn->locked [RmW] pn->state = vcpu_running | |
176 | * | |
177 | * Matches the xchg() from pv_kick_node(). | |
178 | */ | |
b92b8b35 | 179 | smp_store_mb(pn->state, vcpu_halted); |
a23db284 WL |
180 | |
181 | if (!READ_ONCE(node->locked)) | |
182 | pv_wait(&pn->state, vcpu_halted); | |
183 | ||
184 | /* | |
185 | * Reset the vCPU state to avoid unncessary CPU kicking | |
186 | */ | |
187 | WRITE_ONCE(pn->state, vcpu_running); | |
188 | ||
189 | /* | |
190 | * If the locked flag is still not set after wakeup, it is a | |
191 | * spurious wakeup and the vCPU should wait again. However, | |
192 | * there is a pretty high overhead for CPU halting and kicking. | |
193 | * So it is better to spin for a while in the hope that the | |
194 | * MCS lock will be released soon. | |
195 | */ | |
196 | } | |
197 | /* | |
198 | * By now our node->locked should be 1 and our caller will not actually | |
199 | * spin-wait for it. We do however rely on our caller to do a | |
200 | * load-acquire for us. | |
201 | */ | |
202 | } | |
203 | ||
204 | /* | |
205 | * Called after setting next->locked = 1, used to wake those stuck in | |
206 | * pv_wait_node(). | |
207 | */ | |
208 | static void pv_kick_node(struct mcs_spinlock *node) | |
209 | { | |
210 | struct pv_node *pn = (struct pv_node *)node; | |
211 | ||
212 | /* | |
213 | * Note that because node->locked is already set, this actual | |
214 | * mcs_spinlock entry could be re-used already. | |
215 | * | |
216 | * This should be fine however, kicking people for no reason is | |
217 | * harmless. | |
218 | * | |
219 | * See the comment in pv_wait_node(). | |
220 | */ | |
221 | if (xchg(&pn->state, vcpu_running) == vcpu_halted) | |
222 | pv_kick(pn->cpu); | |
223 | } | |
224 | ||
225 | /* | |
226 | * Wait for l->locked to become clear; halt the vcpu after a short spin. | |
227 | * __pv_queued_spin_unlock() will wake us. | |
228 | */ | |
229 | static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node) | |
230 | { | |
231 | struct pv_node *pn = (struct pv_node *)node; | |
232 | struct __qspinlock *l = (void *)lock; | |
233 | struct qspinlock **lp = NULL; | |
234 | int loop; | |
235 | ||
236 | for (;;) { | |
237 | for (loop = SPIN_THRESHOLD; loop; loop--) { | |
238 | if (!READ_ONCE(l->locked)) | |
239 | return; | |
240 | cpu_relax(); | |
241 | } | |
242 | ||
243 | WRITE_ONCE(pn->state, vcpu_halted); | |
244 | if (!lp) { /* ONCE */ | |
245 | lp = pv_hash(lock, pn); | |
246 | /* | |
247 | * lp must be set before setting _Q_SLOW_VAL | |
248 | * | |
249 | * [S] lp = lock [RmW] l = l->locked = 0 | |
250 | * MB MB | |
251 | * [S] l->locked = _Q_SLOW_VAL [L] lp | |
252 | * | |
253 | * Matches the cmpxchg() in __pv_queued_spin_unlock(). | |
254 | */ | |
255 | if (!cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL)) { | |
256 | /* | |
257 | * The lock is free and _Q_SLOW_VAL has never | |
258 | * been set. Therefore we need to unhash before | |
259 | * getting the lock. | |
260 | */ | |
261 | WRITE_ONCE(*lp, NULL); | |
262 | return; | |
263 | } | |
264 | } | |
265 | pv_wait(&l->locked, _Q_SLOW_VAL); | |
266 | ||
267 | /* | |
268 | * The unlocker should have freed the lock before kicking the | |
269 | * CPU. So if the lock is still not free, it is a spurious | |
270 | * wakeup and so the vCPU should wait again after spinning for | |
271 | * a while. | |
272 | */ | |
273 | } | |
274 | ||
275 | /* | |
276 | * Lock is unlocked now; the caller will acquire it without waiting. | |
277 | * As with pv_wait_node() we rely on the caller to do a load-acquire | |
278 | * for us. | |
279 | */ | |
280 | } | |
281 | ||
282 | /* | |
283 | * PV version of the unlock function to be used in stead of | |
284 | * queued_spin_unlock(). | |
285 | */ | |
286 | __visible void __pv_queued_spin_unlock(struct qspinlock *lock) | |
287 | { | |
288 | struct __qspinlock *l = (void *)lock; | |
289 | struct pv_node *node; | |
cba77f03 | 290 | u8 lockval = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0); |
a23db284 WL |
291 | |
292 | /* | |
293 | * We must not unlock if SLOW, because in that case we must first | |
294 | * unhash. Otherwise it would be possible to have multiple @lock | |
295 | * entries, which would be BAD. | |
296 | */ | |
cba77f03 | 297 | if (likely(lockval == _Q_LOCKED_VAL)) |
a23db284 WL |
298 | return; |
299 | ||
cba77f03 WL |
300 | if (unlikely(lockval != _Q_SLOW_VAL)) { |
301 | if (debug_locks_silent) | |
302 | return; | |
303 | WARN(1, "pvqspinlock: lock %p has corrupted value 0x%x!\n", lock, atomic_read(&lock->val)); | |
304 | return; | |
305 | } | |
306 | ||
a23db284 WL |
307 | /* |
308 | * Since the above failed to release, this must be the SLOW path. | |
309 | * Therefore start by looking up the blocked node and unhashing it. | |
310 | */ | |
311 | node = pv_unhash(lock); | |
312 | ||
313 | /* | |
314 | * Now that we have a reference to the (likely) blocked pv_node, | |
315 | * release the lock. | |
316 | */ | |
317 | smp_store_release(&l->locked, 0); | |
318 | ||
319 | /* | |
320 | * At this point the memory pointed at by lock can be freed/reused, | |
321 | * however we can still use the pv_node to kick the CPU. | |
322 | */ | |
323 | if (READ_ONCE(node->state) == vcpu_halted) | |
324 | pv_kick(node->cpu); | |
325 | } | |
326 | /* | |
327 | * Include the architecture specific callee-save thunk of the | |
328 | * __pv_queued_spin_unlock(). This thunk is put together with | |
329 | * __pv_queued_spin_unlock() near the top of the file to make sure | |
330 | * that the callee-save thunk and the real unlock function are close | |
331 | * to each other sharing consecutive instruction cachelines. | |
332 | */ | |
333 | #include <asm/qspinlock_paravirt.h> | |
334 |