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authorStefan Hajnoczi <stefanha@redhat.com>2015-07-28 18:34:09 +0200
committerStefan Hajnoczi <stefanha@redhat.com>2015-07-29 10:02:06 +0100
commitca96ac44dcd290566090b2435bc828fded356ad9 (patch)
treebdf284e97b877f7a9941ac6dd8537ba047677b70 /async.c
parenta076972a4d36381d610a854f0c336507650a1d34 (diff)
downloadqemu-arm-ca96ac44dcd290566090b2435bc828fded356ad9.tar.gz
AioContext: force event loop iteration using BH
The notify_me optimization introduced in commit eabc97797310 ("AioContext: fix broken ctx->dispatching optimization") skips event_notifier_set() calls when the event loop thread is not blocked in ppoll(2). This optimization causes a deadlock if two aio_context_acquire() calls race. notify_me = 0 during the race so the winning thread can enter ppoll(2) unaware that the other thread is waiting its turn to acquire the AioContext. This patch forces ppoll(2) to return by scheduling a BH instead of calling aio_notify(). The following deadlock with virtio-blk dataplane is fixed: qemu ... -object iothread,id=iothread0 \ -drive if=none,id=drive0,file=test.img,... \ -device virtio-blk-pci,iothread=iothread0,drive=drive0 This command-line results in a hang early on without this patch. Thanks to Paolo Bonzini <pbonzini@redhat.com> for investigating this bug with me. Cc: Christian Borntraeger <borntraeger@de.ibm.com> Cc: Cornelia Huck <cornelia.huck@de.ibm.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 1438101249-25166-4-git-send-email-pbonzini@redhat.com Message-Id: <1438014819-18125-3-git-send-email-stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Diffstat (limited to 'async.c')
-rw-r--r--async.c16
1 files changed, 14 insertions, 2 deletions
diff --git a/async.c b/async.c
index 8d23681526..efce14b63a 100644
--- a/async.c
+++ b/async.c
@@ -79,8 +79,10 @@ int aio_bh_poll(AioContext *ctx)
* aio_notify again if necessary.
*/
if (!bh->deleted && atomic_xchg(&bh->scheduled, 0)) {
- if (!bh->idle)
+ /* Idle BHs and the notify BH don't count as progress */
+ if (!bh->idle && bh != ctx->notify_dummy_bh) {
ret = 1;
+ }
bh->idle = 0;
bh->cb(bh->opaque);
}
@@ -230,6 +232,7 @@ aio_ctx_finalize(GSource *source)
{
AioContext *ctx = (AioContext *) source;
+ qemu_bh_delete(ctx->notify_dummy_bh);
thread_pool_free(ctx->thread_pool);
qemu_mutex_lock(&ctx->bh_lock);
@@ -298,8 +301,15 @@ static void aio_timerlist_notify(void *opaque)
static void aio_rfifolock_cb(void *opaque)
{
+ AioContext *ctx = opaque;
+
/* Kick owner thread in case they are blocked in aio_poll() */
- aio_notify(opaque);
+ qemu_bh_schedule(ctx->notify_dummy_bh);
+}
+
+static void notify_dummy_bh(void *opaque)
+{
+ /* Do nothing, we were invoked just to force the event loop to iterate */
}
static void event_notifier_dummy_cb(EventNotifier *e)
@@ -326,6 +336,8 @@ AioContext *aio_context_new(Error **errp)
rfifolock_init(&ctx->lock, aio_rfifolock_cb, ctx);
timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
+ ctx->notify_dummy_bh = aio_bh_new(ctx, notify_dummy_bh, NULL);
+
return ctx;
}