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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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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