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

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Message-ID: <20130623051018.GS6123@two.firstfloor.org>
Date:	Sun, 23 Jun 2013 07:10:18 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Alex Shi <alex.shi@...el.com>
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>,
	Andi Kleen <andi@...stfloor.org>,
	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

> >> 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.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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