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

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

On 2015-08-17 19:18, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@xxxxxxxxxx>
>
> On Mon, 2015-08-17 at 16:25 +0200, Sander Eikelenboom wrote:
> > Monday, August 17, 2015, 4:21:47 PM, you wrote:
> >
> > > On Mon, 2015-08-17 at 09:02 -0500, Jon Christopherson wrote:
> > >> This is very similar to the behavior I am seeing in this bug:
> > >>
> > >> https://bugzilla.kernel.org/show_bug.cgi?id=102911
> >
> > > OK, but have you applied the fix ?
> >
> > > 
> > http://git.kernel.org/cgit/linux/kernel/git/davem/net.git/commit/?id=83fccfc3940c4a2db90fd7e7079f5b465cd8c6af
> >
> > > It will be part of net iteration from David Miller to Linus Torvald.
> >
> >
> > I did have that patch in for my last report.
> > But i don't think he had (looking at the second part of his oops).
> Then can you try following fix as well ?
>
> Thanks !
Running now :)


> [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@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  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);
>                 }
>         }
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