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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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