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    Subject[PATCH][RFC] unbreak generic futex_atomic_cmpxchg_inatomic() on UP
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    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.

    > 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
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    -
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