Re: timers: Move clearing of base::timer_running under base::lock

[Sebastian Andrzej Siewior]

On 2020-12-06 22:40:07 [+0100], Thomas Gleixner wrote:
> syzbot reported KCSAN data races vs. timer_base::timer_running being set to
> NULL without holding base::lock in expire_timers().
> 
> This looks innocent and most reads are clearly not problematic but for a
> non-RT kernel it's completely irrelevant whether the store happens before
> or after taking the lock. For an RT kernel moving the store under the lock
> requires an extra unlock/lock pair in the case that there is a waiter for
> the timer. But that's not the end of the world and definitely not worth the
> trouble of adding boatloads of comments and annotations to the code. Famous
> last words...
> 
> Reported-by: syzbot+aa7c2385d46c5eba0b89@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: syzbot+abea4558531bae1ba9fe@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

One thing I noticed while testing it is that the "corner" case in
timer_sync_wait_running() is quite reliably hit by rcu_preempt
rcu_gp_fqs_loop() -> swait_event_idle_timeout_exclusive() invocation.

Sebastian