This is a note to let you know that I've just added the patch titled

timers: Plug locking race vs. timer migration

to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: timers-plug-locking-race-vs.-timer-migration.patch and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From b831275a3553c32091222ac619cfddd73a5553fb Mon Sep 17 00:00:00 2001

From: Thomas Gleixner tglx@linutronix.de Date: Mon, 24 Oct 2016 11:41:56 +0200 Subject: timers: Plug locking race vs. timer migration

From: Thomas Gleixner tglx@linutronix.de

commit b831275a3553c32091222ac619cfddd73a5553fb upstream.

Linus noticed that lock_timer_base() lacks a READ_ONCE() for accessing the timer flags. As a consequence the compiler is allowed to reload the flags between the initial check for TIMER_MIGRATION and the following timer base computation and the spin lock of the base.

While this has not been observed (yet), we need to make sure that it never happens.

Fixes: 0eeda71bc30d ("timer: Replace timer base by a cpu index") Reported-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Thomas Gleixner tglx@linutronix.de Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1610241711220.4983@nanos Cc: Andrew Morton akpm@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Signed-off-by: Mike Galbraith mgalbraith@suse.de Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

--- kernel/time/timer.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -764,8 +764,15 @@ static struct tvec_base *lock_timer_base __acquires(timer->base->lock) { for (;;) { - u32 tf = timer->flags; struct tvec_base *base; + u32 tf; + + /* + * We need to use READ_ONCE() here, otherwise the compiler + * might re-read @tf between the check for TIMER_MIGRATING + * and spin_lock(). + */ + tf = READ_ONCE(timer->flags);

if (!(tf & TIMER_MIGRATING)) { base = per_cpu_ptr(&tvec_bases, tf & TIMER_CPUMASK);

Patches currently in stable-queue which might be from tglx@linutronix.de are

queue-4.4/prevent-timer-value-0-for-mwaitx.patch queue-4.4/x86-asm-32-make-sync_core-handle-missing-cpuid-on-all-32-bit-kernels.patch queue-4.4/timers-plug-locking-race-vs.-timer-migration.patch queue-4.4/x86-cpu-intel-introduce-macros-for-intel-family-numbers.patch queue-4.4/x86-retpoline-fill-rsb-on-context-switch-for-affected-cpus.patch queue-4.4/time-avoid-undefined-behaviour-in-ktime_add_safe.patch queue-4.4/sched-deadline-use-the-revised-wakeup-rule-for-suspending-constrained-dl-tasks.patch