Messages in this thread Patch in this message | | | Date | Thu, 2 Aug 2007 01:00:21 +0200 (MEST) | From | Mikael Pettersson <> | Subject | [PATCH][RFC] unbreak generic futex_atomic_cmpxchg_inatomic() on UP |
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[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 :->)
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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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