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[PULL,30/62] thread-pool: avoid deadlock in nested aio_poll() calls

Message ID 1407519603-6635-31-git-send-email-kwolf@redhat.com
State New
Headers show

Commit Message

Kevin Wolf Aug. 8, 2014, 5:39 p.m. UTC 
From: Stefan Hajnoczi <stefanha@redhat.com>

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>
---
 thread-pool.c | 6 ++++++
 1 file changed, 6 insertions(+)
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Patch

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;