Re: Linux 4.2-rc6 regression: RIP: e030:[<ffffffff8110fb18>] [<ffffffff8110fb18>] detach_if_pending+0x18/0x80

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, 17 Aug 2015, Eric Dumazet wrote:
> [PATCH] timer: fix a race in __mod_timer()
> 
> lock_timer_base() can not catch following :
> 
> CPU1 ( in __mod_timer()
> timer->flags |= TIMER_MIGRATING;
> spin_unlock(&base->lock);
> base = new_base;
> spin_lock(&base->lock);
> 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 <edumazet@google.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> ---
>  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);

Duh, yes. Picking it up for timers/urgent.

Thanks for spotting it.

       tglx