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From 068000dc97228f85b878634e3d49d3354f5cbafe Mon Sep 17 00:00:00 2001
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From: John Snow <jsnow@redhat.com>
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Date: Fri, 11 Jul 2014 19:07:24 -0500
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Subject: [CHANGE 20/29] aio: fix qemu_bh_schedule() bh->ctx race condition
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To: rhvirt-patches@redhat.com,
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jen@redhat.com
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RH-Author: John Snow <jsnow@redhat.com>
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Message-id: <1405105644-21039-1-git-send-email-jsnow@redhat.com>
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Patchwork-id: 59866
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O-Subject: [RHEL-7.1 qemu-kvm PATCH] aio: fix qemu_bh_schedule() bh->ctx race condition
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Bugzilla: 1116728
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Fam Zheng <famz@redhat.com>
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From: Stefan Hajnoczi <stefanha@redhat.com>
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bz: https://bugzilla.redhat.com/show_bug.cgi?id=1116728
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brew: https://brewweb.devel.redhat.com/taskinfo?taskID=7690195
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upstream: 924fe1293c3e7a3c787bbdfb351e7f168caee3e9
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qemu_bh_schedule() is supposed to be thread-safe at least the first time
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it is called. Unfortunately this is not quite true:
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bh->scheduled = 1;
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aio_notify(bh->ctx);
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Since another thread may run the BH callback once it has been scheduled,
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there is a race condition if the callback frees the BH before
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aio_notify(bh->ctx) has a chance to run.
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Reported-by: Stefan Priebe <s.priebe@profihost.ag>
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
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Tested-by: Stefan Priebe <s.priebe@profihost.ag>
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(cherry picked from commit 924fe1293c3e7a3c787bbdfb351e7f168caee3e9)
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Signed-off-by: John Snow <jsnow@redhat.com>
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---
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async.c | 14 ++++++++++----
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1 file changed, 10 insertions(+), 4 deletions(-)
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Signed-off-by: jen <jen@redhat.com>
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---
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async.c | 14 ++++++++++----
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1 file changed, 10 insertions(+), 4 deletions(-)
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diff --git a/async.c b/async.c
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index 5ce3633..d7ec1ea 100644
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--- a/async.c
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+++ b/async.c
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@@ -117,15 +117,21 @@ void qemu_bh_schedule_idle(QEMUBH *bh)
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void qemu_bh_schedule(QEMUBH *bh)
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{
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+ AioContext *ctx;
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+
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if (bh->scheduled)
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return;
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+ ctx = bh->ctx;
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bh->idle = 0;
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- /* Make sure that idle & any writes needed by the callback are done
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- * before the locations are read in the aio_bh_poll.
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+ /* Make sure that:
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+ * 1. idle & any writes needed by the callback are done before the
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+ * locations are read in the aio_bh_poll.
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+ * 2. ctx is loaded before scheduled is set and the callback has a chance
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+ * to execute.
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*/
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- smp_wmb();
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+ smp_mb();
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bh->scheduled = 1;
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- aio_notify(bh->ctx);
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+ aio_notify(ctx);
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}
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--
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1.9.3
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