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AioContext: fix broken placement of event_notifier_test_and_clear

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event_notifier_test_and_clear must be called before processing events.
Otherwise, an aio_poll could "eat" the notification before the main
I/O thread invokes ppoll().  The main I/O thread then never wakes up.
This is an example of what could happen:

   i/o thread       vcpu thread                     worker thread
   ---------------------------------------------------------------------
   lock_iothread
   notify_me = 1
   ...
   unlock_iothread
                                                     bh->scheduled = 1
                                                     event_notifier_set
                    lock_iothread
                    notify_me = 3
                    ppoll
                    notify_me = 1
                    aio_dispatch
                     aio_bh_poll
                      thread_pool_completion_bh
                                                     bh->scheduled = 1
                                                     event_notifier_set
                     node->io_read(node->opaque)
                      event_notifier_test_and_clear
   ppoll
   *** hang ***

"Tracing" with qemu_clock_get_ns shows pretty much the same behavior as
in the previous bug, so there are no new tricks here---just stare more
at the code until it is apparent.

One could also use a formal model, of course.  The included one shows
this with three processes: notifier corresponds to a QEMU thread pool
worker, temporary_waiter to a VCPU thread that invokes aio_poll(),
waiter to the main I/O thread.  I would be happy to say that the
formal model found the bug for me, but actually I wrote it after the
fact.

This patch is a bit of a big hammer.  The next one optimizes it,
with help (this time for real rather than a posteriori :)) from
another, similar formal model.

Reported-by: Richard W. M. Jones <rjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-6-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Paolo Bonzini 2015-07-21 16:07:52 +02:00 • committed by Stefan Hajnoczi
parent eabc977973
commit 21a03d17f2
4 changed files with 153 additions and 4 deletions
Download patch file Download diff file Expand all files Collapse all files

8
async.c
View file

@ -203,6 +203,8 @@ aio_ctx_check(GSource *source)
QEMUBH *bh;
atomic_and(&ctx->notify_me, ~1);
event_notifier_test_and_clear(&ctx->notifier);
for (bh = ctx->first_bh; bh; bh = bh->next) {
if (!bh->deleted && bh->scheduled) {
return true;
@ -279,6 +281,10 @@ static void aio_rfifolock_cb(void *opaque)
aio_notify(opaque);
}
static void event_notifier_dummy_cb(EventNotifier *e)
{
}
AioContext *aio_context_new(Error **errp)
{
int ret;
@ -293,7 +299,7 @@ AioContext *aio_context_new(Error **errp)
g_source_set_can_recurse(&ctx->source, true);
aio_set_event_notifier(ctx, &ctx->notifier,
(EventNotifierHandler *)
event_notifier_test_and_clear);
event_notifier_dummy_cb);
ctx->thread_pool = NULL;
qemu_mutex_init(&ctx->bh_lock);
rfifolock_init(&ctx->lock, aio_rfifolock_cb, ctx);