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[Qemu-devel] [PULL 02/15] coroutine-lock: add qemu_co_rwlock_downgrade a
From: |
Fam Zheng |
Subject: |
[Qemu-devel] [PULL 02/15] coroutine-lock: add qemu_co_rwlock_downgrade and qemu_co_rwlock_upgrade |
Date: |
Mon, 17 Jul 2017 11:48:12 +0800 |
From: Paolo Bonzini <address@hidden>
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 <address@hidden>
Reviewed-by: Stefan Hajnoczi <address@hidden>
Reviewed-by: Fam Zheng <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Fam Zheng <address@hidden>
---
include/qemu/coroutine.h | 18 ++++++++++++++++++
util/qemu-coroutine-lock.c | 35 +++++++++++++++++++++++++++++++++++
2 files changed, 53 insertions(+)
diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
index a4509bd..9aff9a7 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 fairness if there are no concurrent readers, so
+ * 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, but
+ * 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 b44b5d5..846ff91 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--;
+}
--
2.9.4
- [Qemu-devel] [PULL 00/15] Block and testing patches, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 01/15] qcow2: call CoQueue APIs under CoMutex, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 02/15] coroutine-lock: add qemu_co_rwlock_downgrade and qemu_co_rwlock_upgrade,
Fam Zheng <=
- [Qemu-devel] [PULL 03/15] vdi: make it thread-safe, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 04/15] vpc: make it thread-safe, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 05/15] vvfat: make it thread-safe, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 06/15] qed: move tail of qed_aio_write_main to qed_aio_write_{cow, alloc}, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 08/15] qed: introduce bdrv_qed_init_state, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 07/15] block: invoke .bdrv_drain callback in coroutine context and from AioContext, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 09/15] qed: protect table cache with CoMutex, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 12/15] docker: Don't enable networking as a side-effect of DEBUG=1, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 13/15] docker.py: Drop infile parameter, Fam Zheng, 2017/07/16
- [Qemu-devel] [PULL 11/15] ssh: support I/O from any AioContext, Fam Zheng, 2017/07/16