linux-kernel - Re: [PATCH] spinlock: __raw_spin_is_locked() should return true for UP

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Date:	Tue, 18 Aug 2009 16:20:43 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Kumar Gala <galak@...nel.crashing.org>
cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...utronix.de,
	linuxppc-dev@...abs.org, peterz@...radead.org, rostedt@...dmis.org
Subject: Re: [PATCH] spinlock: __raw_spin_is_locked() should return true for
 UP



On Tue, 18 Aug 2009, Kumar Gala wrote:
>
> For some reason __raw_spin_is_locked() has been returning false for the
> uni-processor, non-CONFIG_DEBUG_SPINLOCK.  The UP + CONFIG_DEBUG_SPINLOCK
> handles this correctly.
> 
> Found this by enabling CONFIG_DEBUG_VM on PPC and hitting always hitting
> a BUG_ON that was testing to make sure the pte_lock was held.
> 
> Signed-off-by: Kumar Gala <galak@...nel.crashing.org>
> ---
> 
> Linus, a fix for 2.6.31

This really isn't all that clear.

The thing is, some people may assert that a lock is held, but others could 
easily be looping until it's not held using something like

	while (spin_is_locked(lock))
		cpu_relax();

so it's hard to tell whether it should return true or false in the case 
where spin-locking simply doesn't exist.

		Linus
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