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authorThomas Gleixner <tglx@linutronix.de>2008-02-23 15:23:57 -0800
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2008-02-23 17:12:15 -0800
commita0c1e9073ef7428a14309cba010633a6cd6719ea (patch)
tree05ce792ddcde92e73d1bae4c8e20f607a2e7db40 /include
parent3e4ab747efa8e78562ec6782b08bbf21a00aba1b (diff)
futex: runtime enable pi and robust functionality
Not all architectures implement futex_atomic_cmpxchg_inatomic(). The default implementation returns -ENOSYS, which is currently not handled inside of the futex guts. Futex PI calls and robust list exits with a held futex result in an endless loop in the futex code on architectures which have no support. Fixing up every place where futex_atomic_cmpxchg_inatomic() is called would add a fair amount of extra if/else constructs to the already complex code. It is also not possible to disable the robust feature before user space tries to register robust lists. Compile time disabling is not a good idea either, as there are already architectures with runtime detection of futex_atomic_cmpxchg_inatomic support. Detect the functionality at runtime instead by calling cmpxchg_futex_value_locked() with a NULL pointer from the futex initialization code. This is guaranteed to fail, but the call of futex_atomic_cmpxchg_inatomic() happens with pagefaults disabled. On architectures, which use the asm-generic implementation or have a runtime CPU feature detection, a -ENOSYS return value disables the PI/robust features. On architectures with a working implementation the call returns -EFAULT and the PI/robust features are enabled. The relevant syscalls return -ENOSYS and the robust list exit code is blocked, when the detection fails. Fixes http://lkml.org/lkml/2008/2/11/149 Originally reported by: Lennart Buytenhek Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Ingo Molnar <mingo@elte.hu> Cc: Lennert Buytenhek <buytenh@wantstofly.org> Cc: Riku Voipio <riku.voipio@movial.fi> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include')
-rw-r--r--include/linux/futex.h1
1 files changed, 1 insertions, 0 deletions
diff --git a/include/linux/futex.h b/include/linux/futex.h
index 90048fb28a3..586ab56a3ec 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -167,6 +167,7 @@ union futex_key {
#ifdef CONFIG_FUTEX
extern void exit_robust_list(struct task_struct *curr);
extern void exit_pi_state_list(struct task_struct *curr);
+extern int futex_cmpxchg_enabled;
#else
static inline void exit_robust_list(struct task_struct *curr)
{