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[Qemu-block] [PATCH 6/6] coroutine-lock: make CoRwlock thread-safe and f
From: |
Paolo Bonzini |
Subject: |
[Qemu-block] [PATCH 6/6] coroutine-lock: make CoRwlock thread-safe and fair |
Date: |
Mon, 13 Feb 2017 19:12:44 +0100 |
This adds a CoMutex around the existing CoQueue. Because the write-side
can just take CoMutex, the old "writer" field is not necessary anymore.
Instead of removing it altogether, count the number of pending writers
during a read-side critical section and forbid further readers from
entering.
Signed-off-by: Paolo Bonzini <address@hidden>
---
include/qemu/coroutine.h | 3 ++-
util/qemu-coroutine-lock.c | 35 ++++++++++++++++++++++++-----------
2 files changed, 26 insertions(+), 12 deletions(-)
diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
index d2de268..e60beaf 100644
--- a/include/qemu/coroutine.h
+++ b/include/qemu/coroutine.h
@@ -204,8 +204,9 @@ bool qemu_co_queue_empty(CoQueue *queue);
typedef struct CoRwlock {
- bool writer;
+ int pending_writer;
int reader;
+ CoMutex mutex;
CoQueue queue;
} CoRwlock;
diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
index b0a554f..6328eed 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -346,16 +346,22 @@ void qemu_co_rwlock_init(CoRwlock *lock)
{
memset(lock, 0, sizeof(*lock));
qemu_co_queue_init(&lock->queue);
+ qemu_co_mutex_init(&lock->mutex);
}
void qemu_co_rwlock_rdlock(CoRwlock *lock)
{
Coroutine *self = qemu_coroutine_self();
- while (lock->writer) {
- qemu_co_queue_wait(&lock->queue, NULL);
+ qemu_co_mutex_lock(&lock->mutex);
+ /* For fairness, wait if a writer is in line. */
+ while (lock->pending_writer) {
+ qemu_co_queue_wait(&lock->queue, &lock->mutex);
}
lock->reader++;
+ qemu_co_mutex_unlock(&lock->mutex);
+
+ /* The rest of the read-side critical section is run without the mutex. */
self->locks_held++;
}
@@ -364,10 +370,13 @@ void qemu_co_rwlock_unlock(CoRwlock *lock)
Coroutine *self = qemu_coroutine_self();
assert(qemu_in_coroutine());
- if (lock->writer) {
- lock->writer = false;
+ if (!lock->reader) {
+ /* The critical section started in qemu_co_rwlock_wrlock. */
qemu_co_queue_restart_all(&lock->queue);
} else {
+ self->locks_held--;
+
+ qemu_co_mutex_lock(&lock->mutex);
lock->reader--;
assert(lock->reader >= 0);
/* Wakeup only one waiting writer */
@@ -375,16 +384,20 @@ void qemu_co_rwlock_unlock(CoRwlock *lock)
qemu_co_queue_next(&lock->queue);
}
}
- self->locks_held--;
+ qemu_co_mutex_unlock(&lock->mutex);
}
void qemu_co_rwlock_wrlock(CoRwlock *lock)
{
- Coroutine *self = qemu_coroutine_self();
-
- while (lock->writer || lock->reader) {
- qemu_co_queue_wait(&lock->queue, NULL);
+ qemu_co_mutex_lock(&lock->mutex);
+ lock->pending_writer++;
+ while (lock->reader) {
+ qemu_co_queue_wait(&lock->queue, &lock->mutex);
}
- lock->writer = true;
- self->locks_held++;
+ lock->pending_writer--;
+
+ /* The rest of the write-side critical section is run with
+ * the mutex taken, so that lock->reader remains zero.
+ * There is no need to update self->locks_held.
+ */
}
--
2.9.3
- [Qemu-block] [PATCH 0/6] Make CoMutex/CoQueue/CoRwlock thread-safe, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 1/6] coroutine-lock: make CoMutex thread-safe, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 2/6] coroutine-lock: add limited spinning to CoMutex, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 3/6] test-aio-multithread: add performance comparison with thread-based mutexes, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 4/6] coroutine-lock: place CoMutex before CoQueue in header, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 5/6] coroutine-lock: add mutex argument to CoQueue APIs, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 6/6] coroutine-lock: make CoRwlock thread-safe and fair,
Paolo Bonzini <=
- Re: [Qemu-block] [PATCH 0/6] Make CoMutex/CoQueue/CoRwlock thread-safe, Fam Zheng, 2017/02/15
- Re: [Qemu-block] [PATCH 0/6] Make CoMutex/CoQueue/CoRwlock thread-safe, Stefan Hajnoczi, 2017/02/16