linux-kernel - Re: [PATCH] kernel: fix data race in put

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Message-ID: <20150918123434.GU3816@twins.programming.kicks-ass.net>
Date:	Fri, 18 Sep 2015 14:34:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	James Hogan <james.hogan@...tec.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Dmitry Vyukov <dvyukov@...gle.com>, ebiederm@...ssion.com,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>, mhocko@...e.cz,
	LKML <linux-kernel@...r.kernel.org>, ktsan@...glegroups.com,
	Kostya Serebryany <kcc@...gle.com>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Hans Boehm <hboehm@...gle.com>
Subject: Re: [PATCH] kernel: fix data race in put_pid

On Fri, Sep 18, 2015 at 01:31:27PM +0100, James Hogan wrote:
> Hi Peter,
> 
> On Fri, Sep 18, 2015 at 11:27:32AM +0200, Peter Zijlstra wrote:
> > On Fri, Sep 18, 2015 at 10:57:32AM +0200, Peter Zijlstra wrote:
> > > diff --git a/arch/metag/include/asm/atomic_lnkget.h b/arch/metag/include/asm/atomic_lnkget.h
> > > index 21c4c268b86c..1bd21c933435 100644
> > > --- a/arch/metag/include/asm/atomic_lnkget.h
> > > +++ b/arch/metag/include/asm/atomic_lnkget.h
> > > @@ -3,7 +3,8 @@
> > >  
> > >  #define ATOMIC_INIT(i)	{ (i) }
> > >  
> > > -#define atomic_set(v, i)		((v)->counter = (i))
> > > +/* XXX: should be LNKSETD ? */
> > > +#define atomic_set(v, i)		WRITE_ONCE((v)->counter, (i))
> > >  
> > >  #include <linux/compiler.h>
> > >  
> > 
> > James?
> 
> It should be fine without a LNKSET. The only reason LNKGET is required
> for atomic_read() is that on certain cores the LNKGET/LNKSET
> instructions went around the cache, but those cores also have write
> through caches, so it shouldn't result in incoherence.
> 
> I've just double checked, and the pipeline seems to handle the linked
> read after write correctly, so e.g. after:
> 
> 0220002c    MOV       D0FrT,#0x5
> b6208042    SETD      [D1Ar3],D0FrT
> ad1080cc    LNKGETD   D0Ar4,[D1Ar3]
> 
> D0Ar4 is correct (0x5).

Thanks! XXX removed.
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