linux-kernel - Re: [PATCH] mutexes: Add CONFIG_DEBUG_MUTEX_FASTPATH=y debug variant to debug SMP races

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Date:	Wed, 4 Dec 2013 13:14:56 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Simon Kirby <sim@...tway.ca>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@...com>,
	Ian Applegate <ia@...udflare.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Lameter <cl@...two.org>,
	Pekka Enberg <penberg@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Mason <chris.mason@...ionio.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] mutexes: Add CONFIG_DEBUG_MUTEX_FASTPATH=y debug variant
 to debug SMP races

On Wed, Dec 4, 2013 at 1:19 AM, Simon Kirby <sim@...tway.ca> wrote:
>
> Meanwhile, I still don't understand how moving the unlock _up_ to cover
> less of the code can solve the race, but I will stare at your long
> explanation more tomorrow.

The lock we're moving up isn't the lock that actually protects the
whole allocation logic (it's the lock that then protects the pipe
contents when a pipe is *used*). So it's a useless lock, and moving it
up is a good idea regardless (because it makes the locks only protect
the parts they are actually *supposed* to protect.

And while extraneous lock wouldn't normally hurt, the sleeping locks
(both mutexes and semaphores) aren't actually safe wrt de-allocation -
they protect anything *inside* the lock, but the lock data structure
itself is accessed racily wrt other lockers (in a way that still
leaves the locked region protected, but not the lock itself). If you
care about details, you can walk through my example.

         Linus
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