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[Qemu-block] [PATCH 02/11] coroutine-lock: add qemu_co_rwlock_downgrade
From: |
Paolo Bonzini |
Subject: |
[Qemu-block] [PATCH 02/11] coroutine-lock: add qemu_co_rwlock_downgrade and qemu_co_rwlock_upgrade |
Date: |
Wed, 31 May 2017 11:43:21 +0200 |
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.
Signed-off-by: Paolo Bonzini <address@hidden>
---
include/qemu/coroutine.h | 19 +++++++++++++++++++
util/qemu-coroutine-lock.c | 35 +++++++++++++++++++++++++++++++++++
2 files changed, 54 insertions(+)
diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
index a4509bd977..0ca96fe3d3 100644
--- a/include/qemu/coroutine.h
+++ b/include/qemu/coroutine.h
@@ -229,6 +229,24 @@ void qemu_co_rwlock_init(CoRwlock *lock);
void qemu_co_rwlock_rdlock(CoRwlock *lock);
/**
+ * Write Locks the CoRwlock from a reader. This is a bit more efficient than
+ * @qemu_co_rwlock_unlock followed by a separate @qemu_co_rwlock_wrlock.
+ * However, if the lock cannot be upgraded immediately, control is transferred
+ * to the caller of the current coroutine. Also, @qemu_co_rwlock_upgrade
+ * only overrides CoRwlock's fairness if there are no concurrent readers;
+ * another writer might run while @qemu_co_rwlock_upgrade blocks.
+ */
+void qemu_co_rwlock_upgrade(CoRwlock *lock);
+
+/**
+ * Downgrades a write-side critical section to a reader. Downgrading with
+ * @qemu_co_rwlock_downgrade never blocks, unlike @qemu_co_rwlock_unlock
+ * followed by @qemu_co_rwlock_rdlock. This makes it more efficient, and
+ * may also sometimes be necessary for correctness.
+ */
+void qemu_co_rwlock_downgrade(CoRwlock *lock);
+
+/**
* Write Locks the mutex. If the lock cannot be taken immediately because
* of a parallel reader, control is transferred to the caller of the current
* coroutine.
diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
index 6328eed26b..bcdcb91ee1 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -387,6 +387,21 @@ 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);
@@ -401,3 +416,23 @@ 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--;
+}
--
2.13.0
- [Qemu-block] [RFC PATCH 00/11] Block layer thread-safety, part 2, Paolo Bonzini, 2017/05/31
- [Qemu-block] [PATCH 01/11] qcow2: call CoQueue APIs under CoMutex, Paolo Bonzini, 2017/05/31
- [Qemu-block] [PATCH 03/11] vdi: make it thread-safe, Paolo Bonzini, 2017/05/31
- [Qemu-block] [PATCH 02/11] coroutine-lock: add qemu_co_rwlock_downgrade and qemu_co_rwlock_upgrade,
Paolo Bonzini <=
- [Qemu-block] [PATCH 04/11] vpc: make it thread-safe, Paolo Bonzini, 2017/05/31
- [Qemu-block] [PATCH 05/11] vvfat: make it thread-safe, Paolo Bonzini, 2017/05/31
- [Qemu-block] [PATCH 06/11] nbd: make it thread-safe, Paolo Bonzini, 2017/05/31
- [Qemu-block] [PATCH 07/11] qed: move tail of qed_aio_write_main to qed_aio_write_{cow, alloc}, Paolo Bonzini, 2017/05/31
- [Qemu-block] [PATCH 08/11] block: invoke .bdrv_drain callback in coroutine context and from AioContext, Paolo Bonzini, 2017/05/31
- [Qemu-block] [PATCH 10/11] sheepdog: add queue_lock, Paolo Bonzini, 2017/05/31