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    SubjectRe: [PATCH][RFC] unbreak generic futex_atomic_cmpxchg_inatomic() on UP
    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.
    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-----
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    > 4KvOt40U1MAPM7g4F/Ps5jk=
    > =68gr
    > -----END PGP SIGNATURE-----
    >
    -
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