linux-kernel - Re: [PATCH 8/8] i386: bitops: smp_mb__{before, after}_clear

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Date:	Tue, 24 Jul 2007 18:31:32 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 8/8] i386: bitops: smp_mb__{before, after}_clear_bit()
 definitions

Jeremy Fitzhardinge wrote:
> Satyam Sharma wrote:
> 
>>Consider this (the above two functions exist only for clear_bit(),
>>the atomic variant, as you already know), the _only_ memory reference
>>we care about is that of the address of the passed bit-string:
>>
>>(1) The compiler must not optimize / elid it (i.e. we need to disallow
>>    compiler optimization for that reference) -- but we've already taken
>>    care of that with the __asm__ __volatile__ and the constraints on
>>    the memory "addr" operand there, and,
>>(2) For the i386, it also includes an implicit memory (CPU) barrier
>>    already.
>>
>>So I /think/ it makes sense to let the compiler optimize _other_ memory
>>references across the call to clear_bit(). There's a difference. I think
>>we'd be safe even if we do this, because the synchronization in callers
>>must be based upon the _passed bit-string_, otherwise _they_ are the
>>ones who're buggy.
>>
>>[ However, elsewhere Jeremy Fitzhardinge mentioned the case of
>>  some callers, for instance, doing a memset() on an alias of
>>  the same bit-string. But again, I think that is dodgy/buggy/
>>  extremely border-line usage on the caller's side itself ...
>>  *unless* the caller is doing that inside a higher-level lock
>>  anyway, in which case he wouldn't be needing to use the
>>  locked variants either ... ]
>>  
> 
> 
> You miss my point.  If you have:
> 
> 	memset(&my_bitmask, 0, sizeof(my_bitmask));
> 	set_bit(my_bitmask, 44);
> 
> Then unless the set_bit has a constraint argument which covers the whole
> of the (multiword) bitmask, the compiler may see fit to interleave the
> memset writes with the set_bit in bad ways.  In other words, plain "+m"
> (*(long *)ptr) won't cut it.  You'd need "+m" (my_bitmask), I think.

That's a valid point, and looks like it is a bug in the existing i386
macros, doesn't it? We should be clobbering addr + BITOP_WORD(nr).

-- 
SUSE Labs, Novell Inc.
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