coroutine-lock: add mutex argument to CoQueue APIs
All that CoQueue needs in order to become thread-safe is help
from an external mutex. Add this to the API.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170213181244.16297-6-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
index 73fe77c..b0a554f 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -40,12 +40,30 @@
QSIMPLEQ_INIT(&queue->entries);
}
-void coroutine_fn qemu_co_queue_wait(CoQueue *queue)
+void coroutine_fn qemu_co_queue_wait(CoQueue *queue, CoMutex *mutex)
{
Coroutine *self = qemu_coroutine_self();
QSIMPLEQ_INSERT_TAIL(&queue->entries, self, co_queue_next);
+
+ if (mutex) {
+ qemu_co_mutex_unlock(mutex);
+ }
+
+ /* 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.
+ */
+ if (mutex) {
+ qemu_co_mutex_lock(mutex);
+ }
}
/**
@@ -335,7 +353,7 @@
Coroutine *self = qemu_coroutine_self();
while (lock->writer) {
- qemu_co_queue_wait(&lock->queue);
+ qemu_co_queue_wait(&lock->queue, NULL);
}
lock->reader++;
self->locks_held++;
@@ -365,7 +383,7 @@
Coroutine *self = qemu_coroutine_self();
while (lock->writer || lock->reader) {
- qemu_co_queue_wait(&lock->queue);
+ qemu_co_queue_wait(&lock->queue, NULL);
}
lock->writer = true;
self->locks_held++;
