linux-kernel - Re: Broken ARM (and powerpc ?) futex wrt memory barriers

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Message-ID: <20090525222744.GA28289@Krystal>
Date:	Mon, 25 May 2009 18:27:44 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Jamie Lokier <jamie@...reable.org>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: Broken ARM (and powerpc ?) futex wrt memory barriers

* Russell King - ARM Linux (linux@....linux.org.uk) wrote:
> On Mon, May 25, 2009 at 05:45:33PM -0400, Mathieu Desnoyers wrote:
> > I think we also have to deal with futexes. See
> > arch/arm/include/asm/futex.h :
> > 
> > 1 - 
> > 
> > #ifdef CONFIG_SMP
> > 
> > #include <asm-generic/futex.h>
> > 
> > #else /* !SMP, we can work around lack of atomic ops by disabling
> > preemption */
> > 
> > (arm-specific code here, seems to deal with futexes)
> > 
> > #endif
> > 
> > -> is it just me or this ifdef condition is the exact opposite of what
> > it should be ? I thought those generic futexes were for UP-only
> > systems...
> 
> No, it is correct.  Look deeper - the generic stuff implements a whole
> load of futex stuff as returning -ENOSYS - it's not implemented.
> 
> What you see in ARM is the UP code implementing the set/add/or/andn/xor
> cmpxchg operations.  However, this code is most certainly not SMP safe
> (and I couldn't persuade the folk writing it to come up with a SMP
> version.)  So, on ARM futex is supported on UP systems, but not SMP
> systems until someone gets around to fixing this.

Ah, yes, you are right. I did not realize ARM SMP support was at such an
early stage. I think there is still some work ahead before those SMP
boards can be fully supported, but I'm glad we're making progress. :)

Mathieu


-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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