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
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Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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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