Re: [PATCH 8/8] i386: bitops: smp_mb__{before, after}_clear_bit()definitions

[Nick Piggin]

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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