linux-kernel - Re: [PATCH 1/2] rwsem: check the lock before cpmxchg in down_write_trylock and rwsem_do

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Date:	Sun, 23 Jun 2013 19:52:12 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Peter Hurley <peter@...leysoftware.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] rwsem: check the lock before cpmxchg in down_write_trylock
 and rwsem_do_wake

On 06/23/2013 01:10 PM, Andi Kleen wrote:
>>>> diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
>>>> > >> index bb1e2cd..052d973 100644
>>>> > >> --- a/include/asm-generic/rwsem.h
>>>> > >> +++ b/include/asm-generic/rwsem.h
>>>> > >> @@ -70,11 +70,11 @@ static inline void __down_write(struct
>>>> > >> rw_semaphore *sem)
>>>> > >>
>>>> > >>   static inline int __down_write_trylock(struct rw_semaphore *sem)
>>>> > >>   {
>>>> > >> -    long tmp;
>>>> > >> +    if (unlikely(&sem->count != RWSEM_UNLOCKED_VALUE))
>>> > >                      ^^^^^^^^^^^
>>> > > 
>>> > > This is probably not what you want.
>>> > > 
>> > 
>> > this function logical is quite simple. check the sem->count before
>> > cmpxchg is no harm this logical.
>> > 
>> > So could you like to tell us what should we want?
> You are comparing the address, not the value. Remove the &
> This was a nop too.

ops, So stupid I am! :(

-- 
Thanks
    Alex
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