linux-kernel - Re: [PATCH] futex: Ensure get_futex_key

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Date:	Sat, 18 Oct 2014 00:33:00 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	linux-kernel@...r.kernel.org,
	Matteo Franchin <Matteo.Franchin@....com>,
	Davidlohr Bueso <davidlohr@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a
 barrier

On Fri, 2014-10-17 at 17:38 +0100, Catalin Marinas wrote:
> Commit b0c29f79ecea (futexes: Avoid taking the hb->lock if there's
> nothing to wake up) changes the futex code to avoid taking a lock when
> there are no waiters. This code has been subsequently fixed in commit
> 11d4616bd07f (futex: revert back to the explicit waiter counting code).
> Both the original commit and the fix-up rely on get_futex_key_refs() to
> always imply a barrier.
> 
> However, for private futexes, none of the cases in the switch statement
> of get_futex_key_refs() would be hit and the function completes without
> a memory barrier as required before checking the "waiters" in
> futex_wake() -> hb_waiters_pending(). 

Good catch, glad I ran into this thread (my email recently changed).
Private process futex (PTHREAD_PROCESS_PRIVATE) have no reference on an
inode or mm so it would need the explicit barrier in those cases.

> The consequence is a race with a
> thread waiting on a futex on another CPU, allowing the waker thread to
> read "waiters == 0" while the waiter thread to have read "futex_val ==
> locked" (in kernel).

Yeah missing wakeups are a strong sign of a problem with the
hb_waiters_pending() side.

> Without this fix, the problem (user space deadlocks) can be seen with
> Android bionic's mutex implementation on an arm64 multi-cluster system.
> Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> Reported-by: Matteo Franchin <Matteo.Franchin@....com>
> Fixes: b0c29f79ecea (futexes: Avoid taking the hb->lock if there's nothing to wake up)
> Cc: <stable@...r.kernel.org>
> Cc: Davidlohr Bueso <davidlohr@...com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Darren Hart <dvhart@...ux.intel.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  kernel/futex.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 815d7af2ffe8..f3a3a071283c 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -343,6 +343,8 @@ static void get_futex_key_refs(union futex_key *key)
>  	case FUT_OFF_MMSHARED:
>  		futex_get_mm(key); /* implies MB (B) */
>  		break;
> +	default:
> +		smp_mb(); /* explicit MB (B) */
>  	}

Should we comment that this default is for the private futex case?
Otherwise:

Acked-by: Davidlohr Bueso <dave@...olabs.net>

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