linux-kernel - Re: [PATCH] futex: avoid race between requeue and wake

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Date:	Tue, 8 Apr 2014 15:30:07 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Stancek <jstancek@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Davidlohr Bueso <davidlohr@...com>,
	Ingo Molnar <mingo@...nel.org>,
	Larry Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH] futex: avoid race between requeue and wake

On Tue, Apr 8, 2014 at 2:02 PM, Jan Stancek <jstancek@...hat.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.

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..

Still hoping for comments from PeterZ and Davidlohr.

                  Linus

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