linux-kernel - Re: [PATCH] futex: cmpxchg_futex_value

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Message-ID: <20110309160839.283e241a@mschwide.boeblingen.de.ibm.com>
Date:	Wed, 9 Mar 2011 16:08:39 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Darren Hart <darren@...art.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Matt Turner <mattst88@...il.com>,
	Russell King <linux@....linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Tony Luck <tony.luck@...el.com>,
	Michal Simek <monstr@...str.eu>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	"David S. Miller" <davem@...emloft.net>,
	Chris Metcalf <cmetcalf@...era.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] futex: cmpxchg_futex_value_locked API change

On Wed, 9 Mar 2011 03:25:50 -0800
Michel Lespinasse <walken@...gle.com> wrote:

> On Tue, Mar 08, 2011 at 09:17:11PM +0100, Thomas Gleixner wrote:
> > On Sun, 6 Mar 2011, Michel Lespinasse wrote:
> > > The cmpxchg_futex_value_locked API was funny in that it returned either
> > > the original, user-exposed futex value OR an error code such as -EFAULT.
> > > This was confusing at best, and could be a source of livelocks in places
> > > that retry the cmpxchg_futex_value_locked after trying to fix the issue
> > > by running fault_in_user_writeable().
> > > 
> > > This change makes the cmpxchg_futex_value_locked API more similar to the
> > > get_futex_value_locked one, returning an error code and updating the
> > > original value through a reference argument.
> > 
> > Ack.
> > 
> > >  static inline int
> > > -futex_atomic_cmpxchg_inatomic(int __user *uaddr, int oldval, int newval)
> > > +futex_atomic_cmpxchg_inatomic(int *uval, int __user *uaddr,
> > > +			      int oldval, int newval)
> > 
> > Can we please change the arguments to u32 while at it? The futex core
> > code uses u32 all over the place so those inlines should do the same.
> 
> All right. Attaching updated version of the diff. I also fixed the s390
> issue reported by Martin Schwidefsky.

The inline assmeblies look good now but I get this compile error on s390:

  CC      arch/s390/lib/uaccess_pt.o
arch/s390/lib/uaccess_pt.c:373: error: conflicting types for 'futex_atomic_cmpxchg_pt'
arch/s390/lib/uaccess.h:21: note: previous declaration of 'futex_atomic_cmpxchg_pt' was here
arch/s390/lib/uaccess_pt.c:403: warning: initialization from incompatible pointer type
make[1]: *** [arch/s390/lib/uaccess_pt.o] Error 1

The trouble is this part of the patch:

-int futex_atomic_cmpxchg_pt(int __user *uaddr, int oldval, int newval)
+int futex_atomic_cmpxchg_pt(int *uval, int __user *uaddr,
+                           int oldval, int newval)

You need to change these "int"s to "u32"s.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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