kernel/locking/qspinlock_paravirt.h

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