From 068000dc97228f85b878634e3d49d3354f5cbafe Mon Sep 17 00:00:00 2001 From: John Snow 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 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 RH-Acked-by: Stefan Hajnoczi RH-Acked-by: Fam Zheng From: Stefan Hajnoczi 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 Signed-off-by: Stefan Hajnoczi Reviewed-by: Paolo Bonzini Tested-by: Stefan Priebe (cherry picked from commit 924fe1293c3e7a3c787bbdfb351e7f168caee3e9) Signed-off-by: John Snow --- async.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) Signed-off-by: jen --- 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