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

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Message-Id: <BB2762BC-C760-4D4C-BDCF-76EFD3E1B18D@kernel.crashing.org>
Date:	Tue, 18 Aug 2009 20:17:28 -0500
From:	Kumar Gala <galak@...nel.crashing.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...utronix.de,
	linuxppc-dev@...abs.org, peterz@...radead.org
Subject: Re: [PATCH] spinlock: __raw_spin_is_locked() should return true for UP


On Aug 18, 2009, at 7:07 PM, Steven Rostedt wrote:

> On Tue, 18 Aug 2009, Linus Torvalds wrote:
>>
>>> Thinking about it, UP probably should have spin_is_locked always  
>>> return
>>> false, but if you want to make sure you are not in a critical  
>>> section
>>> with the lock not held, then use assert_spin_locked, which on UP  
>>> should be
>>> a nop.
>>
>> That's what we do. That said, I also think we should generally try to
>> avoid the kind of code that depends on spin_is_locked always  
>> returning
>> false, for the same reason we should try to avoid any code that  
>> depends on
>> it always returning true.
>
> Perhaps we can deprecate spin_is_locked and replace it with
> "expect_spin_locked" and "expect_spin_unlocked" which on SMP would  
> return
> true and false respectively if the lock was locked. But both would  
> always
> return true on UP.
>
> Or do some thing similar, that would remove the ambiguity on UP.

I agree its a little too easy to abuse spin_is_locked.  However we  
should be consistent between spin_is_locked on UP between with and  
without CONFIG_DEBUG_SPINLOCK enabled.  How much of this do we want to  
try and address in .31?

The PPC test really should be using assert_spin_locked and I'll send a  
patch to Ben for that.

- k
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