qemu

qemu-coroutine-lock.c (13806B)

---

 /*
  * coroutine queues and locks
  *
  * Copyright (c) 2011 Kevin Wolf <kwolf@redhat.com>
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
  * in the Software without restriction, including without limitation the rights
  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  * copies of the Software, and to permit persons to whom the Software is
  * furnished to do so, subject to the following conditions:
  *
  * The above copyright notice and this permission notice shall be included in
  * all copies or substantial portions of the Software.
  *
  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  * THE SOFTWARE.
  *
  * The lock-free mutex implementation is based on OSv
  * (core/lfmutex.cc, include/lockfree/mutex.hh).
  * Copyright (C) 2013 Cloudius Systems, Ltd.
  */
 
 #include "qemu/osdep.h"
 #include "qemu/coroutine.h"
 #include "qemu/coroutine_int.h"
 #include "qemu/processor.h"
 #include "qemu/queue.h"
 #include "block/aio.h"
 #include "trace.h"
 
 void qemu_co_queue_init(CoQueue *queue)
 {
     QSIMPLEQ_INIT(&queue->entries);
 }
 
 void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock,
                                           CoQueueWaitFlags flags)
 {
     Coroutine *self = qemu_coroutine_self();
     if (flags & CO_QUEUE_WAIT_FRONT) {
         QSIMPLEQ_INSERT_HEAD(&queue->entries, self, co_queue_next);
     } else {
         QSIMPLEQ_INSERT_TAIL(&queue->entries, self, co_queue_next);
     }
 
     if (lock) {
         qemu_lockable_unlock(lock);
     }
 
     /* There is no race condition here.  Other threads will call
      * aio_co_schedule on our AioContext, which can reenter this
      * coroutine but only after this yield and after the main loop
      * has gone through the next iteration.
      */
     qemu_coroutine_yield();
     assert(qemu_in_coroutine());
 
     /* TODO: OSv implements wait morphing here, where the wakeup
      * primitive automatically places the woken coroutine on the
      * mutex's queue.  This avoids the thundering herd effect.
      * This could be implemented for CoMutexes, but not really for
      * other cases of QemuLockable.
      */
     if (lock) {
         qemu_lockable_lock(lock);
     }
 }
 
 bool qemu_co_enter_next_impl(CoQueue *queue, QemuLockable *lock)
 {
     Coroutine *next;
 
     next = QSIMPLEQ_FIRST(&queue->entries);
     if (!next) {
         return false;
     }
 
     QSIMPLEQ_REMOVE_HEAD(&queue->entries, co_queue_next);
     if (lock) {
         qemu_lockable_unlock(lock);
     }
     aio_co_wake(next);
     if (lock) {
         qemu_lockable_lock(lock);
     }
     return true;
 }
 
 bool coroutine_fn qemu_co_queue_next(CoQueue *queue)
 {
     /* No unlock/lock needed in coroutine context.  */
     return qemu_co_enter_next_impl(queue, NULL);
 }
 
 void qemu_co_enter_all_impl(CoQueue *queue, QemuLockable *lock)
 {
     while (qemu_co_enter_next_impl(queue, lock)) {
         /* just loop */
     }
 }
 
 void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue)
 {
     /* No unlock/lock needed in coroutine context.  */
     qemu_co_enter_all_impl(queue, NULL);
 }
 
 bool qemu_co_queue_empty(CoQueue *queue)
 {
     return QSIMPLEQ_FIRST(&queue->entries) == NULL;
 }
 
 /* The wait records are handled with a multiple-producer, single-consumer
  * lock-free queue.  There cannot be two concurrent pop_waiter() calls
  * because pop_waiter() can only be called while mutex->handoff is zero.
  * This can happen in three cases:
  * - in qemu_co_mutex_unlock, before the hand-off protocol has started.
  *   In this case, qemu_co_mutex_lock will see mutex->handoff == 0 and
  *   not take part in the handoff.
  * - in qemu_co_mutex_lock, if it steals the hand-off responsibility from
  *   qemu_co_mutex_unlock.  In this case, qemu_co_mutex_unlock will fail
  *   the cmpxchg (it will see either 0 or the next sequence value) and
  *   exit.  The next hand-off cannot begin until qemu_co_mutex_lock has
  *   woken up someone.
  * - in qemu_co_mutex_unlock, if it takes the hand-off token itself.
  *   In this case another iteration starts with mutex->handoff == 0;
  *   a concurrent qemu_co_mutex_lock will fail the cmpxchg, and
  *   qemu_co_mutex_unlock will go back to case (1).
  *
  * The following functions manage this queue.
  */
 typedef struct CoWaitRecord {
     Coroutine *co;
     QSLIST_ENTRY(CoWaitRecord) next;
 } CoWaitRecord;
 
 static void coroutine_fn push_waiter(CoMutex *mutex, CoWaitRecord *w)
 {
     w->co = qemu_coroutine_self();
     QSLIST_INSERT_HEAD_ATOMIC(&mutex->from_push, w, next);
 }
 
 static void move_waiters(CoMutex *mutex)
 {
     QSLIST_HEAD(, CoWaitRecord) reversed;
     QSLIST_MOVE_ATOMIC(&reversed, &mutex->from_push);
     while (!QSLIST_EMPTY(&reversed)) {
         CoWaitRecord *w = QSLIST_FIRST(&reversed);
         QSLIST_REMOVE_HEAD(&reversed, next);
         QSLIST_INSERT_HEAD(&mutex->to_pop, w, next);
     }
 }
 
 static CoWaitRecord *pop_waiter(CoMutex *mutex)
 {
     CoWaitRecord *w;
 
     if (QSLIST_EMPTY(&mutex->to_pop)) {
         move_waiters(mutex);
         if (QSLIST_EMPTY(&mutex->to_pop)) {
             return NULL;
         }
     }
     w = QSLIST_FIRST(&mutex->to_pop);
     QSLIST_REMOVE_HEAD(&mutex->to_pop, next);
     return w;
 }
 
 static bool has_waiters(CoMutex *mutex)
 {
     return QSLIST_EMPTY(&mutex->to_pop) || QSLIST_EMPTY(&mutex->from_push);
 }
 
 void qemu_co_mutex_init(CoMutex *mutex)
 {
     memset(mutex, 0, sizeof(*mutex));
 }
 
 static void coroutine_fn qemu_co_mutex_wake(CoMutex *mutex, Coroutine *co)
 {
     /* Read co before co->ctx; pairs with smp_wmb() in
      * qemu_coroutine_enter().
      */
     smp_read_barrier_depends();
     mutex->ctx = co->ctx;
     aio_co_wake(co);
 }
 
 static void coroutine_fn qemu_co_mutex_lock_slowpath(AioContext *ctx,
                                                      CoMutex *mutex)
 {
     Coroutine *self = qemu_coroutine_self();
     CoWaitRecord w;
     unsigned old_handoff;
 
     trace_qemu_co_mutex_lock_entry(mutex, self);
     push_waiter(mutex, &w);
 
     /* This is the "Responsibility Hand-Off" protocol; a lock() picks from
      * a concurrent unlock() the responsibility of waking somebody up.
      */
     old_handoff = qatomic_mb_read(&mutex->handoff);
     if (old_handoff &&
         has_waiters(mutex) &&
         qatomic_cmpxchg(&mutex->handoff, old_handoff, 0) == old_handoff) {
         /* There can be no concurrent pops, because there can be only
          * one active handoff at a time.
          */
         CoWaitRecord *to_wake = pop_waiter(mutex);
         Coroutine *co = to_wake->co;
         if (co == self) {
             /* We got the lock ourselves!  */
             assert(to_wake == &w);
             mutex->ctx = ctx;
             return;
         }
 
         qemu_co_mutex_wake(mutex, co);
     }
 
     qemu_coroutine_yield();
     trace_qemu_co_mutex_lock_return(mutex, self);
 }
 
 void coroutine_fn qemu_co_mutex_lock(CoMutex *mutex)
 {
     AioContext *ctx = qemu_get_current_aio_context();
     Coroutine *self = qemu_coroutine_self();
     int waiters, i;
 
     /* Running a very small critical section on pthread_mutex_t and CoMutex
      * shows that pthread_mutex_t is much faster because it doesn't actually
      * go to sleep.  What happens is that the critical section is shorter
      * than the latency of entering the kernel and thus FUTEX_WAIT always
      * fails.  With CoMutex there is no such latency but you still want to
      * avoid wait and wakeup.  So introduce it artificially.
      */
     i = 0;
 retry_fast_path:
     waiters = qatomic_cmpxchg(&mutex->locked, 0, 1);
     if (waiters != 0) {
         while (waiters == 1 && ++i < 1000) {
             if (qatomic_read(&mutex->ctx) == ctx) {
                 break;
             }
             if (qatomic_read(&mutex->locked) == 0) {
                 goto retry_fast_path;
             }
             cpu_relax();
         }
         waiters = qatomic_fetch_inc(&mutex->locked);
     }
 
     if (waiters == 0) {
         /* Uncontended.  */
         trace_qemu_co_mutex_lock_uncontended(mutex, self);
         mutex->ctx = ctx;
     } else {
         qemu_co_mutex_lock_slowpath(ctx, mutex);
     }
     mutex->holder = self;
     self->locks_held++;
 }
 
 void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex)
 {
     Coroutine *self = qemu_coroutine_self();
 
     trace_qemu_co_mutex_unlock_entry(mutex, self);
 
     assert(mutex->locked);
     assert(mutex->holder == self);
     assert(qemu_in_coroutine());
 
     mutex->ctx = NULL;
     mutex->holder = NULL;
     self->locks_held--;
     if (qatomic_fetch_dec(&mutex->locked) == 1) {
         /* No waiting qemu_co_mutex_lock().  Pfew, that was easy!  */
         return;
     }
 
     for (;;) {
         CoWaitRecord *to_wake = pop_waiter(mutex);
         unsigned our_handoff;
 
         if (to_wake) {
             qemu_co_mutex_wake(mutex, to_wake->co);
             break;
         }
 
         /* Some concurrent lock() is in progress (we know this because
          * mutex->locked was >1) but it hasn't yet put itself on the wait
          * queue.  Pick a sequence number for the handoff protocol (not 0).
          */
         if (++mutex->sequence == 0) {
             mutex->sequence = 1;
         }
 
         our_handoff = mutex->sequence;
         qatomic_mb_set(&mutex->handoff, our_handoff);
         if (!has_waiters(mutex)) {
             /* The concurrent lock has not added itself yet, so it
              * will be able to pick our handoff.
              */
             break;
         }
 
         /* Try to do the handoff protocol ourselves; if somebody else has
          * already taken it, however, we're done and they're responsible.
          */
         if (qatomic_cmpxchg(&mutex->handoff, our_handoff, 0) != our_handoff) {
             break;
         }
     }
 
     trace_qemu_co_mutex_unlock_return(mutex, self);
 }
 
 struct CoRwTicket {
     bool read;
     Coroutine *co;
     QSIMPLEQ_ENTRY(CoRwTicket) next;
 };
 
 void qemu_co_rwlock_init(CoRwlock *lock)
 {
     qemu_co_mutex_init(&lock->mutex);
     lock->owners = 0;
     QSIMPLEQ_INIT(&lock->tickets);
 }
 
 /* Releases the internal CoMutex.  */
 static void coroutine_fn qemu_co_rwlock_maybe_wake_one(CoRwlock *lock)
 {
     CoRwTicket *tkt = QSIMPLEQ_FIRST(&lock->tickets);
     Coroutine *co = NULL;
 
     /*
      * Setting lock->owners here prevents rdlock and wrlock from
      * sneaking in between unlock and wake.
      */
 
     if (tkt) {
         if (tkt->read) {
             if (lock->owners >= 0) {
                 lock->owners++;
                 co = tkt->co;
             }
         } else {
             if (lock->owners == 0) {
                 lock->owners = -1;
                 co = tkt->co;
             }
         }
     }
 
     if (co) {
         QSIMPLEQ_REMOVE_HEAD(&lock->tickets, next);
         qemu_co_mutex_unlock(&lock->mutex);
         aio_co_wake(co);
     } else {
         qemu_co_mutex_unlock(&lock->mutex);
     }
 }
 
 void coroutine_fn qemu_co_rwlock_rdlock(CoRwlock *lock)
 {
     Coroutine *self = qemu_coroutine_self();
 
     qemu_co_mutex_lock(&lock->mutex);
     /* For fairness, wait if a writer is in line.  */
     if (lock->owners == 0 || (lock->owners > 0 && QSIMPLEQ_EMPTY(&lock->tickets))) {
         lock->owners++;
         qemu_co_mutex_unlock(&lock->mutex);
     } else {
         CoRwTicket my_ticket = { true, self };
 
         QSIMPLEQ_INSERT_TAIL(&lock->tickets, &my_ticket, next);
         qemu_co_mutex_unlock(&lock->mutex);
         qemu_coroutine_yield();
         assert(lock->owners >= 1);
 
         /* Possibly wake another reader, which will wake the next in line.  */
         qemu_co_mutex_lock(&lock->mutex);
         qemu_co_rwlock_maybe_wake_one(lock);
     }
 
     self->locks_held++;
 }
 
 void coroutine_fn qemu_co_rwlock_unlock(CoRwlock *lock)
 {
     Coroutine *self = qemu_coroutine_self();
 
     assert(qemu_in_coroutine());
     self->locks_held--;
 
     qemu_co_mutex_lock(&lock->mutex);
     if (lock->owners > 0) {
         lock->owners--;
     } else {
         assert(lock->owners == -1);
         lock->owners = 0;
     }
 
     qemu_co_rwlock_maybe_wake_one(lock);
 }
 
 void coroutine_fn qemu_co_rwlock_downgrade(CoRwlock *lock)
 {
     qemu_co_mutex_lock(&lock->mutex);
     assert(lock->owners == -1);
     lock->owners = 1;
 
     /* Possibly wake another reader, which will wake the next in line.  */
     qemu_co_rwlock_maybe_wake_one(lock);
 }
 
 void coroutine_fn qemu_co_rwlock_wrlock(CoRwlock *lock)
 {
     Coroutine *self = qemu_coroutine_self();
 
     qemu_co_mutex_lock(&lock->mutex);
     if (lock->owners == 0) {
         lock->owners = -1;
         qemu_co_mutex_unlock(&lock->mutex);
     } else {
         CoRwTicket my_ticket = { false, qemu_coroutine_self() };
 
         QSIMPLEQ_INSERT_TAIL(&lock->tickets, &my_ticket, next);
         qemu_co_mutex_unlock(&lock->mutex);
         qemu_coroutine_yield();
         assert(lock->owners == -1);
     }
 
     self->locks_held++;
 }
 
 void coroutine_fn qemu_co_rwlock_upgrade(CoRwlock *lock)
 {
     qemu_co_mutex_lock(&lock->mutex);
     assert(lock->owners > 0);
     /* For fairness, wait if a writer is in line.  */
     if (lock->owners == 1 && QSIMPLEQ_EMPTY(&lock->tickets)) {
         lock->owners = -1;
         qemu_co_mutex_unlock(&lock->mutex);
     } else {
         CoRwTicket my_ticket = { false, qemu_coroutine_self() };
 
         lock->owners--;
         QSIMPLEQ_INSERT_TAIL(&lock->tickets, &my_ticket, next);
         qemu_co_rwlock_maybe_wake_one(lock);
         qemu_coroutine_yield();
         assert(lock->owners == -1);
     }
 }