From 667221c10d05a22573dae1928880bca525f8cd20 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Thu, 29 Jun 2017 15:27:40 +0200
Subject: coroutine-lock: add qemu_co_rwlock_downgrade and
qemu_co_rwlock_upgrade
These functions are more efficient in the presence of contention.
qemu_co_rwlock_downgrade also guarantees not to block, which may
be useful in some algorithms too.
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170629132749.997-3-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
---
util/qemu-coroutine-lock.c | 35 +++++++++++++++++++++++++++++++++++
1 file changed, 35 insertions(+)
(limited to 'util')
diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
index b44b5d55eb..846ff9167f 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -402,6 +402,21 @@ void qemu_co_rwlock_unlock(CoRwlock *lock)
qemu_co_mutex_unlock(&lock->mutex);
}
+void qemu_co_rwlock_downgrade(CoRwlock *lock)
+{
+ Coroutine *self = qemu_coroutine_self();
+
+ /* lock->mutex critical section started in qemu_co_rwlock_wrlock or
+ * qemu_co_rwlock_upgrade.
+ */
+ assert(lock->reader == 0);
+ lock->reader++;
+ qemu_co_mutex_unlock(&lock->mutex);
+
+ /* The rest of the read-side critical section is run without the mutex. */
+ self->locks_held++;
+}
+
void qemu_co_rwlock_wrlock(CoRwlock *lock)
{
qemu_co_mutex_lock(&lock->mutex);
@@ -416,3 +431,23 @@ void qemu_co_rwlock_wrlock(CoRwlock *lock)
* There is no need to update self->locks_held.
*/
}
+
+void qemu_co_rwlock_upgrade(CoRwlock *lock)
+{
+ Coroutine *self = qemu_coroutine_self();
+
+ qemu_co_mutex_lock(&lock->mutex);
+ assert(lock->reader > 0);
+ lock->reader--;
+ lock->pending_writer++;
+ while (lock->reader) {
+ qemu_co_queue_wait(&lock->queue, &lock->mutex);
+ }
+ lock->pending_writer--;
+
+ /* The rest of the write-side critical section is run with
+ * the mutex taken, similar to qemu_co_rwlock_wrlock. Do
+ * not account for the lock twice in self->locks_held.
+ */
+ self->locks_held--;
+}
--
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