Re: Add memory barrier when waiting on futex

[Darren Hart]

On Mon, 2013-11-25 at 15:39 +0100, Peter Zijlstra wrote:
> On Mon, Nov 25, 2013 at 01:15:17PM +0000, Ma, Xindong wrote:
> > We encountered following panic several times:
> 
> > [   74.671982] BUG: unable to handle kernel NULL pointer dereference at 00000008
> > [   74.672101] IP: [<c129bb27>] wake_futex+0x47/0x80
> 
> > [   74.674144]  [<c129bc29>] futex_wake+0xc9/0x110
> > [   74.674195]  [<c129da0b>] do_futex+0xeb/0x950
> > [   74.674484]  [<c129e30b>] SyS_futex+0x9b/0x140
> > [   74.674582]  [<c195a718>] syscall_call+0x7/0xb
> > 
> > On smp systems, setting current task to q->task in queue_me() may
> > not visible immediately to another cpu, some times this will
> > cause panic in wake_futex(). Adding memory barrier to avoid this.
> > 
> > Signed-off-by: Leon Ma <xindong.ma@xxxxxxxxx>
> > Signed-off-by: xiaobing tu <xiaobing.tu@xxxxxxxxx>
> > ---
> >  kernel/futex.c |    1 +
> >  1 files changed, 1 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/futex.c b/kernel/futex.c
> > index 80ba086..792cd41 100644
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -1529,6 +1529,7 @@ static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
> >  	plist_node_init(&q->list, prio);
> >  	plist_add(&q->list, &hb->chain);
> >  	q->task = current;
> > +	smp_mb();
> >  	spin_unlock(&hb->lock);
> >  }
> 
> This is wrong, because an uncommented barrier is wrong per definition.
> 
> This is further wrong because wake_futex() is always called with
> hb->lock held, and since queue_me also has hb->lock held, this is in
> fact guaranteed.

Indeed. Leon Ma, would you have a test case to share? I'm interested to
see how this was possible.

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel


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