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From 068000dc97228f85b878634e3d49d3354f5cbafe Mon Sep 17 00:00:00 2001
From: John Snow <jsnow@redhat.com>
Date: Fri, 11 Jul 2014 19:07:24 -0500
Subject: [CHANGE 20/29] aio: fix qemu_bh_schedule() bh->ctx race condition
To: rhvirt-patches@redhat.com,
    jen@redhat.com

RH-Author: John Snow <jsnow@redhat.com>
Message-id: <1405105644-21039-1-git-send-email-jsnow@redhat.com>
Patchwork-id: 59866
O-Subject: [RHEL-7.1 qemu-kvm PATCH] aio: fix qemu_bh_schedule() bh->ctx race condition
Bugzilla: 1116728
RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: Fam Zheng <famz@redhat.com>

From: Stefan Hajnoczi <stefanha@redhat.com>

bz: https://bugzilla.redhat.com/show_bug.cgi?id=1116728
brew: https://brewweb.devel.redhat.com/taskinfo?taskID=7690195
upstream: 924fe1293c3e7a3c787bbdfb351e7f168caee3e9

qemu_bh_schedule() is supposed to be thread-safe at least the first time
it is called.  Unfortunately this is not quite true:

  bh->scheduled = 1;
  aio_notify(bh->ctx);

Since another thread may run the BH callback once it has been scheduled,
there is a race condition if the callback frees the BH before
aio_notify(bh->ctx) has a chance to run.

Reported-by: Stefan Priebe <s.priebe@profihost.ag>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Stefan Priebe <s.priebe@profihost.ag>
(cherry picked from commit 924fe1293c3e7a3c787bbdfb351e7f168caee3e9)
Signed-off-by: John Snow <jsnow@redhat.com>
---
 async.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

Signed-off-by: jen <jen@redhat.com>
---
 async.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/async.c b/async.c
index 5ce3633..d7ec1ea 100644
--- a/async.c
+++ b/async.c
@@ -117,15 +117,21 @@ void qemu_bh_schedule_idle(QEMUBH *bh)
 
 void qemu_bh_schedule(QEMUBH *bh)
 {
+    AioContext *ctx;
+
     if (bh->scheduled)
         return;
+    ctx = bh->ctx;
     bh->idle = 0;
-    /* Make sure that idle & any writes needed by the callback are done
-     * before the locations are read in the aio_bh_poll.
+    /* Make sure that:
+     * 1. idle & any writes needed by the callback are done before the
+     *    locations are read in the aio_bh_poll.
+     * 2. ctx is loaded before scheduled is set and the callback has a chance
+     *    to execute.
      */
-    smp_wmb();
+    smp_mb();
     bh->scheduled = 1;
-    aio_notify(bh->ctx);
+    aio_notify(ctx);
 }
 
 
-- 
1.9.3