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

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Date:	Wed, 11 Dec 2013 10:03:00 -0500
From:	Waiman Long <waiman.long@...com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Simon Kirby <sim@...tway.ca>,
	Peter Zijlstra <peterz@...radead.org>,
	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 12/03/2013 03:52 AM, Ingo Molnar wrote:
> *
> I think we try that, to make mutexes safer in general.
>
> Can you see a way to do that fairly cheaply?
>
> I can see two approaches, both rather radical:
>
> 1)
>
> Eliminate mutex->count and (ab-)use mutex->wait_lock as 'the' mutex
> lock: with TICKET_SLOWPATH_FLAG used to denote waiters or so and care
> taken to not use it as a 'real' spinlock but use the raw accessors.
>
> This would still allow a good mutex_lock() fastpath, as it would
> essentially become spin_trylock() with an asm goto slow path helper
> perhaps.
>
> Doing this would have various advantages:
>
>   - we'd eliminate much (all?) of per arch mutex code
>   - we'd share spinlock and mutex low level implementations
>   - we'd reduce struct mutex size by 4 bytes
>
> It's still early in the morning so I might be missing something
> trivial though - this sounds suspiciously too easy ;-) Having a proper
> mutex slowpath might not be so easy without changing the spinlock
> code.
>
> 2)
>
> Another method would be to do the opposite: eliminate mutex->wait_lock
> [for the non-debug case] and do everything via mutex->count and
> mutex->owner.
>
> This saves even more space and potentially enables a tighter slowpath.
>
> It probably won't hurt the massively parallel case, as we already do
> smart MCS locking via mutex->spin_mlock.
>
> So I'd argue for #2. (Assuming it addresses the problem)
>
> Thanks,
>
> 	Ingo
>
>

I also think that #2 is safer as messing with spinlock code can be 
risky. However, #2 probably won't work for architectures that use the 
generic mutex-xchg.h fastpath. Currently the following architectures use 
mutex-xchg.h - unicore32, arc, arm and hexagon. Is there a reason why 
they cannot be converted to use mutex-dec.h instead?

-Longman


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