From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Tue, 15 Jul 2014 14:44:26 +0000 (+0200)
Subject: thread-pool: avoid deadlock in nested aio_poll() calls
X-Git-Tag: TizenStudio_2.0_p2.3.2~208^2~656^2~32
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=3c80ca158c96ff902a30883a8933e755988948b1;p=sdk%2Femulator%2Fqemu.git

thread-pool: avoid deadlock in nested aio_poll() calls

The thread pool has a race condition if two elements complete before
thread_pool_completion_bh() runs:

  If element A's callback waits for element B using aio_poll() it will
  deadlock since pool->completion_bh is not marked scheduled when the
  nested aio_poll() runs.

Fix this by marking the BH scheduled while thread_pool_completion_bh()
is executing.  This way any nested aio_poll() loops will enter
thread_pool_completion_bh() and complete the remaining elements.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---

diff --git a/thread-pool.c b/thread-pool.c
index 4cfd078..23888dc 100644
--- a/thread-pool.c
+++ b/thread-pool.c
@@ -185,6 +185,12 @@ restart:
             QLIST_REMOVE(elem, all);
             /* Read state before ret.  */
             smp_rmb();
+
+            /* Schedule ourselves in case elem->common.cb() calls aio_poll() to
+             * wait for another request that completed at the same time.
+             */
+            qemu_bh_schedule(pool->completion_bh);
+
             elem->common.cb(elem->common.opaque, elem->ret);
             qemu_aio_release(elem);
             goto restart;