From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Gleixner Subject: Re: Linux 4.2-rc6 regression: RIP: e030:[] [] detach_if_pending+0x18/0x80 Date: Mon, 17 Aug 2015 21:13:35 +0200 (CEST) Message-ID: References: <7c39e6332aae572372108d5fd68c503b@eikelenboom.it> <1439412094.29802.21.camel@edumazet-glaptop2.roam.corp.google.com> <0de9f30086ee2e662bdafee61d24b3c1@eikelenboom.it> <20150812.144040.1151964094247340464.davem@davemloft.net> <1439419279.29802.25.camel@edumazet-glaptop2.roam.corp.google.com> <3b284689aaa1573610c0e388e66e756a@eikelenboom.it> <1439591965.7507.6.camel@edumazet-glaptop2.roam.corp.google.com> <963062024.20150817110956@eikelenboom.it> <1439818633.7258.16.camel@edumazet-glaptop2.roam.corp.google.com> <196897465.20150817154802@eikelenboom.it> <55D1E97B.6090309@jons.org> <1439821307.7258.20.camel@edumazet-glaptop2.roam.corp.google.com> <373726164.20150817162534@eikelenboom.it> <1439831928.32680.11.camel@edumazet-glaptop2.roam.corp.google.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: Sander Eikelenboom , Jon Christopherson , David Miller , linux-kernel@vger.kernel.org, netdev@vger.kernel.org, xen-devel@lists.xen.org, david.vrabel@citrix.com To: Eric Dumazet Return-path: In-Reply-To: <1439831928.32680.11.camel@edumazet-glaptop2.roam.corp.google.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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 > Cc: Thomas Gleixner > --- > 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