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[Qemu-devel] [PATCH 04/18] aio: make ctx->list_lock a QemuLockCnt, subsu
From: |
Paolo Bonzini |
Subject: |
[Qemu-devel] [PATCH 04/18] aio: make ctx->list_lock a QemuLockCnt, subsuming ctx->walking_bh |
Date: |
Thu, 6 Aug 2015 15:36:02 +0200 |
This will make it possible to walk the list of bottom halves without
holding the AioContext lock---and in turn to call bottom half
handlers without holding the lock.
Signed-off-by: Paolo Bonzini <address@hidden>
---
async.c | 28 ++++++++++++----------------
include/block/aio.h | 12 +++++-------
2 files changed, 17 insertions(+), 23 deletions(-)
diff --git a/async.c b/async.c
index 7e97614..69b76dc 100644
--- a/async.c
+++ b/async.c
@@ -50,12 +50,12 @@ QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void
*opaque)
.cb = cb,
.opaque = opaque,
};
- qemu_mutex_lock(&ctx->list_lock);
+ qemu_lockcnt_lock(&ctx->list_lock);
bh->next = ctx->first_bh;
/* Make sure that the members are ready before putting bh into list */
smp_wmb();
ctx->first_bh = bh;
- qemu_mutex_unlock(&ctx->list_lock);
+ qemu_lockcnt_unlock(&ctx->list_lock);
return bh;
}
@@ -65,13 +65,11 @@ int aio_bh_poll(AioContext *ctx)
QEMUBH *bh, **bhp, *next;
int ret;
- ctx->walking_bh++;
+ qemu_lockcnt_inc(&ctx->list_lock);
ret = 0;
- for (bh = ctx->first_bh; bh; bh = next) {
- /* Make sure that fetching bh happens before accessing its members */
- smp_read_barrier_depends();
- next = bh->next;
+ for (bh = atomic_rcu_read(&ctx->first_bh); bh; bh = next) {
+ next = atomic_rcu_read(&bh->next);
/* The atomic_xchg is paired with the one in qemu_bh_schedule. The
* implicit memory barrier ensures that the callback sees all writes
* done by the scheduling thread. It also ensures that the scheduling
@@ -88,11 +86,8 @@ int aio_bh_poll(AioContext *ctx)
}
}
- ctx->walking_bh--;
-
/* remove deleted bhs */
- if (!ctx->walking_bh) {
- qemu_mutex_lock(&ctx->list_lock);
+ if (qemu_lockcnt_dec_and_lock(&ctx->list_lock)) {
bhp = &ctx->first_bh;
while (*bhp) {
bh = *bhp;
@@ -103,7 +98,7 @@ int aio_bh_poll(AioContext *ctx)
bhp = &bh->next;
}
}
- qemu_mutex_unlock(&ctx->list_lock);
+ qemu_lockcnt_unlock(&ctx->list_lock);
}
return ret;
@@ -234,7 +229,8 @@ aio_ctx_finalize(GSource *source)
thread_pool_free(ctx->thread_pool);
- qemu_mutex_lock(&ctx->list_lock);
+ qemu_lockcnt_lock(&ctx->list_lock);
+ assert(!qemu_lockcnt_count(&ctx->list_lock));
while (ctx->first_bh) {
QEMUBH *next = ctx->first_bh->next;
@@ -244,12 +240,12 @@ aio_ctx_finalize(GSource *source)
g_free(ctx->first_bh);
ctx->first_bh = next;
}
- qemu_mutex_unlock(&ctx->list_lock);
+ qemu_lockcnt_unlock(&ctx->list_lock);
aio_set_event_notifier(ctx, &ctx->notifier, NULL);
event_notifier_cleanup(&ctx->notifier);
rfifolock_destroy(&ctx->lock);
- qemu_mutex_destroy(&ctx->list_lock);
+ qemu_lockcnt_destroy(&ctx->list_lock);
timerlistgroup_deinit(&ctx->tlg);
}
@@ -318,7 +314,7 @@ AioContext *aio_context_new(Error **errp)
(EventNotifierHandler *)
event_notifier_dummy_cb);
ctx->thread_pool = NULL;
- qemu_mutex_init(&ctx->list_lock);
+ qemu_lockcnt_init(&ctx->list_lock);
rfifolock_init(&ctx->lock, NULL, NULL);
timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
diff --git a/include/block/aio.h b/include/block/aio.h
index 026f6c1..3988d28 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -88,17 +88,15 @@ struct AioContext {
*/
uint32_t notify_me;
- /* lock to protect between bh's adders and deleter */
- QemuMutex list_lock;
+ /* A lock to protect between bh's adders and deleter, and to ensure
+ * that no callbacks are removed while we're walking and dispatching
+ * them.
+ */
+ QemuLockCnt list_lock;
/* Anchor of the list of Bottom Halves belonging to the context */
struct QEMUBH *first_bh;
- /* A simple lock used to protect the first_bh list, and ensure that
- * no callbacks are removed while we're walking and dispatching callbacks.
- */
- int walking_bh;
-
/* Used by aio_notify.
*
* "notified" is used to avoid expensive event_notifier_test_and_clear
--
2.4.3
- [Qemu-devel] [RFC PATCH 00/18] Fine-grained AioContext critical sections, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 02/18] aio: rename bh_lock to list_lock, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 03/18] qemu-thread: introduce QemuLockCnt, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 04/18] aio: make ctx->list_lock a QemuLockCnt, subsuming ctx->walking_bh,
Paolo Bonzini <=
- [Qemu-devel] [PATCH 06/18] aio-posix: remove walking_handlers, protecting AioHandler list with list_lock, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 09/18] aio: push aio_context_acquire/release down to dispatching, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 10/18] async: optimize aio_bh_poll, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 16/18] quorum: split quorum_fifo_aio_cb from quorum_aio_cb, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 08/18] aio: document locking, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 14/18] block: explicitly acquire aiocontext in timers that need it, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 05/18] aio: tweak walking in dispatch phase, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 15/18] quorum: use atomics for rewrite_count, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 18/18] aio: update locking documentation, Paolo Bonzini, 2015/08/06
- [Qemu-devel] [PATCH 01/18] iothread: release iothread around aio_poll, Paolo Bonzini, 2015/08/06