From b49c0f24cf6744a3f4fd09289fe7cade349dead5 Mon Sep 17 00:00:00 2001 From: Nicolas Pitre Date: Tue, 20 Nov 2007 17:20:29 +0100 Subject: [ARM] 4659/1: remove possibilities for spurious false negative with __kuser_cmpxchg The ARM __kuser_cmpxchg routine is meant to implement an atomic cmpxchg in user space. It however can produce spurious false negative if a processor exception occurs in the middle of the operation. Normally this is not a problem since cmpxchg is typically called in a loop until it succeeds to implement an atomic increment for example. Some use cases which don't involve a loop require that the operation be 100% reliable though. This patch changes the implementation so to reattempt the operation after an exception has occurred in the critical section rather than abort it. Here's a simple program to test the fix (don't use CONFIG_NO_HZ in your kernel as this depends on a sufficiently high interrupt rate): #include typedef int (__kernel_cmpxchg_t)(int oldval, int newval, int *ptr); #define __kernel_cmpxchg (*(__kernel_cmpxchg_t *)0xffff0fc0) int main() { int i, x = 0; for (i = 0; i < 100000000; i++) { int v = x; if (__kernel_cmpxchg(v, v+1, &x)) printf("failed at %d: %d vs %d\n", i, v, x); } printf("done with %d vs %d\n", i, x); return 0; } Signed-off-by: Nicolas Pitre Signed-off-by: Russell King --- arch/arm/kernel/traps.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) (limited to 'arch/arm/kernel/traps.c') diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c index 5a52dd7f97fe..c34db4e868fa 100644 --- a/arch/arm/kernel/traps.c +++ b/arch/arm/kernel/traps.c @@ -509,7 +509,7 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs) * existence. Don't ever use this from user code. */ case 0xfff0: - { + for (;;) { extern void do_DataAbort(unsigned long addr, unsigned int fsr, struct pt_regs *regs); unsigned long val; @@ -545,7 +545,6 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs) up_read(&mm->mmap_sem); /* simulate a write access fault */ do_DataAbort(addr, 15 + (1 << 11), regs); - return -1; } #endif -- cgit v1.2.2