Messages in this thread Patch in this message | | | Date | Wed, 9 Apr 2014 07:46:30 -0400 (EDT) | From | Jan Stancek <> | Subject | Re: [PATCH] futex: avoid race between requeue and wake |
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----- Original Message ----- > From: "Linus Torvalds" <torvalds@linux-foundation.org> > To: "Jan Stancek" <jstancek@redhat.com> > Cc: "Peter Zijlstra" <peterz@infradead.org>, "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>, "Srikar > Dronamraju" <srikar@linux.vnet.ibm.com>, "Davidlohr Bueso" <davidlohr@hp.com>, "Ingo Molnar" <mingo@kernel.org>, > "Larry Woodman" <lwoodman@redhat.com> > Sent: Wednesday, 9 April, 2014 12:30:07 AM > Subject: Re: [PATCH] futex: avoid race between requeue and wake > > On Tue, Apr 8, 2014 at 2:02 PM, Jan Stancek <jstancek@redhat.com> wrote: > > > > I ran reproducer with following change on s390x system, where this > > can be reproduced usually within seconds: > > > > diff --git a/kernel/futex.c b/kernel/futex.c > > index 67dacaf..9150ffd 100644 > > --- a/kernel/futex.c > > +++ b/kernel/futex.c > > @@ -1095,6 +1095,7 @@ static int unlock_futex_pi(u32 __user *uaddr, u32 > > uval) > > static inline void > > double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket > > *hb2) > > { > > + hb_waiters_inc(hb2); > > if (hb1 <= hb2) { > > spin_lock(&hb1->lock); > > if (hb1 < hb2) > > @@ -1111,6 +1112,7 @@ double_unlock_hb(struct futex_hash_bucket *hb1, > > struct futex_hash_bucket *hb2) > > spin_unlock(&hb1->lock); > > if (hb1 != hb2) > > spin_unlock(&hb2->lock); > > + hb_waiters_dec(hb2); > > } > > > > /* > > > > Reproducer is running without failures over an hour now and > > made ~1.4 million iterations.
I let this version run over night on single s390x system, there were no failures.
> > Ok, that's encouraging. That is the smallest patch I could come up > with, but as mentioned, it's not optimal. We only need it for > futex_requeue(), but if we do it there we'd have to handle all the > different error cases (there's only one call to double_lock_hb(), but > due to the error cases there's four calls to double_unlock_hb(). > > I'm not sure how much we care. The simple patch basically adds two > (unnecessary) atomics to the futex_wake_op() path. I don't know how > critical that path is - not as critical as the regular "futex_wake()", > I'd expect, but I guess pthread_cond_signal() is the main user. > > So I'll have to leave this decision to the futex people. But the > attached slightly more complex patch *may* be the better one. > > May I bother you to test this one too? I really think that > futex_requeue() is the only user that should need this, so doing it > there rather than in double_[un]lock_hb() should be slightly more > optimal, but who knows what I've missed. We clearly *all* missed this > race back when the ordering rules were documented..
I'm running reproducer with this patch applied on 3 systems: - two s390x systems where this can be reproduced within seconds - x86_64 Intel(R) Xeon(R) CPU E5240 @ 3.00GHz, where I could reproduce it on average in ~3 minutes. It's running without failure over 4 hours now.
Regards, Jan
> > Still hoping for comments from PeterZ and Davidlohr. > > Linus >
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