To: linux-kernel@vger.kernel.org
From: Stephen Hemminger <shemminger@linux-foundation.org>
Subject: Re: Question regarding mutex locking
Date: Wed, 28 Nov 2007 15:33:12 -0800
Organization: Linux Foundation
Message-ID: <20071128153312.14144472@freepuppy.rosehill>
References: <474CF06C.8020208@lwfinger.net>
	<jemysy2y3y.fsf@sykes.suse.de>
	<474D8C22.7010902@lwfinger.net>
	<474DEF79.1080900@o2.pl>
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On Wed, 28 Nov 2007 23:45:13 +0100
Jarek Poplawski <jarkao2@o2.pl> wrote:

> Larry Finger wrote, On 11/28/2007 04:41 PM:
> 
> > Andreas Schwab wrote:
> >> Larry Finger <Larry.Finger@lwfinger.net> writes:
> >>
> >>> If a particular routine needs to lock a mutex, but it may be entered with that mutex already locked,
> >>> would the following code be SMP safe?
> >>>
> >>> hold_lock = mutex_trylock()
> >>>
> >>> ...
> >>>
> >>> if (hold_lock)
> >>> 	mutex_unlock()
> >> When two CPUs may enter the critical region at the same time, what is
> >> the point of the mutex?  Also, the first CPU may unlock the mutex while
> >> the second one is still inside the critical region.
> > 
> > Thank you for that answer. I think that I'm finally beginning to understand.
> 
> Probably it would be faster without these "...", which look like
> no man's land...
> 
> hold_lock = mutex_trylock()
> if (hold_lock) {
> 	/* SMP safe */
> 	...
>  	mutex_unlock()
> } else {
> 	/* SMP unsafe */
> 	...
> 	/* maybe try again after some break or check */
> }
> 
> Regards,
> Jarek P.

WTF are you teaching a lesson on how NOT to do locking?

Any code which has this kind of convoluted dependency on conditional
locking is fundamentally broken.

-- 
Stephen Hemminger <shemminger@linux-foundation.org>

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