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

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Message-ID: <20160901102006.ye5uhbhwyqijgxvc@localhost>
Date:   Thu, 1 Sep 2016 11:20:07 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Vladimir Murzin <vladimir.murzin@....com>
Cc:     Kenneth Lee <liguozhu@...ilicon.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] [bugfix] replace unnessary ldax with common ldr

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@...ilicon.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