linux-kernel - Re: [PATCH][RFC] unbreak generic futex_atomic_cmpxchg

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Date:	Thu, 4 Oct 2007 10:07:31 +0200 (CEST)
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Mikael Pettersson <mikpe@...uu.se>
Cc:	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	riku.voipio@....fi, buytenh@...tstofly.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH][RFC] unbreak generic futex_atomic_cmpxchg_inatomic() on
 UP

On Thu, 4 Oct 2007, Mikael Pettersson wrote:
> On Wed, 3 Oct 2007 22:33:46 +0300, Riku Voipio wrote:
> > What's the state of this patch? I can confirm tst-robust1
> > from glibc testsuite locks a armv5 machine hard. With this patch
> > applied, the test succeeds.
> 
> There were no comments from any Linux arch or futex maintainer.

Probably because it wasn't posted to linux-arch?

> Because of that I intend to submit an ARM-only patch when 2.6.23
> has been released.
> 
> /Mikael
> 
> > > The higher-end archs (x86, sparc64, ppc64, etc) provide fully-functional
> > > asm/futex.h implementations, but a number of archs (alpha, arm, arm26,
> > > avr32, blackfin, cris, h8300, m32r, m68k, mk68knommu, sh64, sparc, um,
> > > v850, and xtensa) use asm-generic/futex.h, which makes robust futexes
> > > horribly broken on them. There have also been reports recently that PI
> > > futexes are broken due to the generic futex_atomic_cmpxchg_inatomic()
> > > just being an -ENOSYS stub.
> > 
> > This an effective local DOS bug on the affected architectures, too.. 
> > 
> > > The patch below implements the generic futex_atomic_cmpxchg_inatomic() in
> > > terms of __copy_{from,to}_user_inatomic() and preempt_{disable,enable}().
> > > It obviously doesn't support SMP, but UP-only support should go a long
> > > way for users of the affected archs.
> > > 
> > > I'm using this patch now and it has allowed me to build and use glibc-2.4
> > > with NPTL on ARM (glibc-2.4-11.src.rpm from FC5 + ARM fixes).
> > > (Finally I can ditch LinuxThreads :->)
> > > 
> > > Comments?
> > > 
> > > /Mikael
> > > 
> > > --- linux-2.6.22/include/asm-generic/futex.h.~1~	2007-02-04 19:44:54.000000000 +0100
> > > +++ linux-2.6.22/include/asm-generic/futex.h	2007-08-01 19:03:43.000000000 +0200
> > > @@ -4,6 +4,7 @@
> > >  #ifdef __KERNEL__
> > >  
> > >  #include <linux/futex.h>
> > > +#include <linux/preempt.h>
> > >  #include <asm/errno.h>
> > >  #include <asm/uaccess.h>
> > >  
> > > @@ -52,7 +53,34 @@ futex_atomic_op_inuser (int encoded_op, 
> > >  static inline int
> > >  futex_atomic_cmpxchg_inatomic(int __user *uaddr, int oldval, int newval)
> > > {
> > > +#ifdef CONFIG_SMP
> > >  	return -ENOSYS;
> > > +#else
> > > +	int curval, ret;
> > > +
> > > +	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(int)))
> > > +		return -EFAULT;
> > > +
> > > +	preempt_disable();
> > > +
> > > +	ret = -EFAULT;
> > > +	if (__copy_from_user_inatomic(&curval, uaddr, sizeof(int)))
> > > +		goto out;
> > > +
> > > +	ret = curval;
> > > +	if (curval != oldval)
> > > +		goto out;
> > > +
> > > +	ret = -EFAULT;
> > > +	if (__copy_to_user_inatomic(uaddr, &newval, sizeof(int)))
> > > +		goto out;
> > > +
> > > +	ret = newval;
> > > +
> > > + out:
> > > +	preempt_enable();
> > > +	return ret;
> > > +#endif
> > >  }
> > >  
> > >  #endif
> > > -
> > 
> > - -- 
> > "rm -rf" only sounds scary if you don't have backups
> > -----BEGIN PGP SIGNATURE-----
> > Version: GnuPG v1.4.6 (GNU/Linux)
> > 
> > iD8DBQFHA+6aibPvMsrqrwMRAlMuAKCBf7qpD2dETgU+RgnDG4ArVvFp3gCgrewq
> > 4KvOt40U1MAPM7g4F/Ps5jk=
> > =68gr
> > -----END PGP SIGNATURE-----
> > 
> -
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Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds
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