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

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

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 !

[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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