Messages in this thread | | | From | Alexander Potapenko <> | Date | Tue, 2 Apr 2019 13:31:57 +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 10:59 AM Alexander Potapenko <glider@google.com> wrote: > > 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"); Updated the patch. I took the liberty of not adding the symbolic names to reduce the diff. If you think that's necessary I can do that in a separate patch. > > ? > > > > And the very same for _all_ other sites touched in this patch. > > > > -- > Alexander Potapenko > Software Engineer > > Google Germany GmbH > Erika-Mann-Straße, 33 > 80636 München > > Geschäftsführer: Paul Manicle, Halimah DeLaine Prado > Registergericht und -nummer: Hamburg, HRB 86891 > Sitz der Gesellschaft: Hamburg
-- Alexander Potapenko Software Engineer
Google Germany GmbH Erika-Mann-Straße, 33 80636 München
Geschäftsführer: Paul Manicle, Halimah DeLaine Prado Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg
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