Messages in this thread | | | From | Alexander Potapenko <> | Date | Tue, 2 Apr 2019 10:59:05 +0200 | Subject | Re: [PATCH] x86/asm: use memory clobber in bitops that touch arbitrary memory |
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On Tue, Apr 2, 2019 at 9:27 AM Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Mon, Apr 01, 2019 at 06:24:08PM +0200, Alexander Potapenko wrote: > > diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h > > index d153d570bb04..20e4950827d9 100644 > > --- a/arch/x86/include/asm/bitops.h > > +++ b/arch/x86/include/asm/bitops.h > > @@ -111,7 +111,7 @@ clear_bit(long nr, volatile unsigned long *addr) > > } else { > > asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0" > > : BITOP_ADDR(addr) > > - : "Ir" (nr)); > > + : "Ir" (nr) : "memory"); > > } > > } > > clear_bit() doesn't have a return value, so why are we now still using > "+m" output ? You're right, "m" should suffice. I'll update the patch. By the way, now that we've added the memory barriers we can probably remove the barrier() call from __clear_bit_unlock() and update the comment accordingly. For clear_bit() the memory barrier is missing on the IS_IMMEDIATE(nr) path, so this one should be kept as is.
> AFAICT the only reason we did that was to clobber the variable, which > you've (afaiu correctly) argued to be incorrect. > > So whould we not write this as: > > asm volatile (LOCK_PREFIX __ASM_SIZE(btr) " %[nr], %[addr]" > : : [addr] "m" (*addr), [nr] "Ir" (nr) > : "memory"); > > ? > > And the very same for _all_ other sites touched in this patch.
-- Alexander Potapenko Software Engineer
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