block: only call aio_poll on the current thread's AioContext

aio_poll is not thread safe; for example bdrv_drain can hang if
the last in-flight I/O operation is completed in the I/O thread after
the main thread has checked bs->in_flight.

The bug remains latent as long as all of it is called within
aio_context_acquire/aio_context_release, but this will change soon.

To fix this, if bdrv_drain is called from outside the I/O thread,
signal the main AioContext through a dummy bottom half.  The event
loop then only runs in the I/O thread.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1477565348-5458-18-git-send-email-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/block.c b/block.c
index fbe485c4f8..a17baab1d0 100644
--- a/block.c
+++ b/block.c
@@ -2090,7 +2090,9 @@ int bdrv_reopen_multiple(AioContext *ctx, BlockReopenQueue *bs_queue, Error **er
 
     assert(bs_queue != NULL);
 
+    aio_context_release(ctx);
     bdrv_drain_all();
+    aio_context_acquire(ctx);
 
     QSIMPLEQ_FOREACH(bs_entry, bs_queue, entry) {
         if (bdrv_reopen_prepare(&bs_entry->state, bs_queue, &local_err)) {