From d0023a1448abdcc892b8bca631e74bb1888efd02 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 17 Aug 2015 10:18:48 -0700 Subject: [PATCH] timer: Write timer->flags atomically lock_timer_base() cannot prevent the following : CPU1 ( in __mod_timer() timer->flags |= TIMER_MIGRATING; spin_unlock(&base->lock); base = new_base; spin_lock(&base->lock); // The next line clears TIMER_MIGRATING timer->flags &= ~TIMER_BASEMASK; CPU2 (in lock_timer_base()) see timer base is cpu0 base spin_lock_irqsave(&base->lock, *flags); if (timer->flags == tf) return base; // oops, wrong base timer->flags |= base->cpu // too late We must write timer->flags in one go, otherwise we can fool other cpus. Fixes: bc7a34b8b9eb ("timer: Reduce timer migration overhead if disabled") Signed-off-by: Eric Dumazet Cc: Jon Christopherson Cc: David Miller Cc: xen-devel@lists.xen.org Cc: david.vrabel@citrix.com Cc: Sander Eikelenboom Link: http://lkml.kernel.org/r/1439831928.32680.11.camel@edumazet-glaptop2.roam.corp.google.com Signed-off-by: Thomas Gleixner Cc: Thomas Gleixner --- kernel/time/timer.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/time/timer.c b/kernel/time/timer.c index 5e097fa9faf7..84190f02b521 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -807,8 +807,8 @@ __mod_timer(struct timer_list *timer, unsigned long expires, spin_unlock(&base->lock); base = new_base; spin_lock(&base->lock); - timer->flags &= ~TIMER_BASEMASK; - timer->flags |= base->cpu; + WRITE_ONCE(timer->flags, + (timer->flags & ~TIMER_BASEMASK) | base->cpu); } } -- 2.39.5