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Subject[PATCH][RFC] unbreak generic futex_atomic_cmpxchg_inatomic() on UP
[Resend. I messed up the To: line the first time]

As has been reported recently by Lennert Buytenhek, robust futexes
are broken on ARM:

>If you're also running into glibc's tst-robust1 test suite test
>locking up your ARM machine, you're probably running into the fact
>that asm-arm/futex.h includes asm-generic/futex.h, and
>asm-generic/futex.h defines futex_atomic_cmpxchg_inatomic() to
>return -ENOSYS. This causes handle_futex_death() to loop forever.

I can confirm this statement: building glibc-2.4 with NPTL on
ARM hangs the kernel when the test suite reaches tst-robust1.

The problem is that kernel/futex.c expects futex_atomic_cmpxchg_inatomic()
to return -EFAULT or the new value. It doesn't expect -ENOSYS at all, and
generally -ENOSYS causes the futex code to loop, hanging the kernel.

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.

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 :->)



--- 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;
+ 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;

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