Re: [PATCH] [bugfix] replace unnessary ldax with common ldr

From	Catalin Marinas <catalin.marinas@arm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] [bugfix] replace unnessary ldax with common ldr
Date	2016-09-01 12:30 +0200
Message-ID	<scxpU-vd-33@gated-at.bofh.it> (permalink)
References	<sbKyR-15N-3@gated-at.bofh.it> <sbNdo-2Ps-27@gated-at.bofh.it> <scdUd-2XV-7@gated-at.bofh.it>
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On Wed, Aug 31, 2016 at 02:30:40PM +0100, Vladimir Murzin wrote:
> On 30/08/16 10:07, Catalin Marinas wrote:
> > On Tue, Aug 30, 2016 at 02:35:31PM +0800, Kenneth Lee wrote:
> >> (add comment for the previous mail, sorry for the duplication)
> >>
> >> There is no store_ex pairing with this load_ex. It is not necessary and
> >> gave wrong hint to the cache system.
> >>
> >> Signed-off-by: Kenneth Lee <liguozhu@hisilicon.com>
> >> ---
> >>  arch/arm64/include/asm/spinlock.h | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
> >> index c85e96d..3334c4f 100644
> >> --- a/arch/arm64/include/asm/spinlock.h
> >> +++ b/arch/arm64/include/asm/spinlock.h
> >> @@ -63,7 +63,7 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
> >>  	 */
> >>  "	sevl\n"
> >>  "2:	wfe\n"
> >> -"	ldaxrh	%w2, %4\n"
> >> +"	ldrh	%w2, %4\n"
> >>  "	eor	%w1, %w2, %w0, lsr #16\n"
> >>  "	cbnz	%w1, 2b\n"
> >>  	/* We got the lock. Critical section starts here. */
> > 
> > This is needed because the arch_spin_unlock() code only uses an STLR
> > without an explicit SEV (like we have on AArch32). An event is
> > automatically generated when the exclusive monitor is cleared by STLR.
> > But without setting it with a load exclusive in arch_spin_lock() (even
> > though it does not acquire the lock), there won't be anything to clear,
> > hence no event to be generated. In this case, the WFE would wait
> > indefinitely.
> 
> Maybe worth to add this as a comment, no?

Yes, we just need to find someone to send a patch ;).

-- 
Catalin

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[PATCH] [bugfix] replace unnessary ldax with common ldr  Kenneth Lee <liguozhu@hisilicon.com> - 2016-08-30 08:20 +0200
  Re: [PATCH] [bugfix] replace unnessary ldax with common ldr Catalin Marinas <catalin.marinas@arm.com> - 2016-08-30 11:10 +0200
    Re: [PATCH] [bugfix] replace unnessary ldax with common ldr Vladimir Murzin <vladimir.murzin@arm.com> - 2016-08-31 15:40 +0200
      Re: [PATCH] [bugfix] replace unnessary ldax with common ldr Catalin Marinas <catalin.marinas@arm.com> - 2016-09-01 12:30 +0200

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