From 91f4ab056d85d23fa6955927fdeb1558673e8cd1 Mon Sep 17 00:00:00 2001 From: Hirokazu Takata Date: Mon, 28 Nov 2005 13:43:58 -0800 Subject: [PATCH] [PATCH] m32r: Fix sys_tas() syscall This patch fixes a deadlock problem of the m32r SMP kernel. In the m32r kernel, sys_tas() system call is provided as a test-and-set function for userspace, for backward compatibility. In some multi-threading application program, deadlocks were rarely caused at sys_tas() funcion. Such a deadlock was caused due to a collision of __pthread_lock() and __pthread_unlock() operations. The "tas" syscall is repeatedly called by pthread_mutex_lock() to get a lock, while a lock variable's value is not 0. On the other hand, pthead_mutex_unlock() sets the lock variable to 0 for unlocking. In the previous implementation of sys_tas() routine, there was a possibility that a unlock operation was ignored in the following case: - Assume a lock variable (*addr) was equal to 1 before sys_tas() execution. - __pthread_unlock() operation is executed by the other processor and the lock variable (*addr) is set to 0, between a read operation ("oldval = *addr;") and the following write operation ("*addr = 1;") during a execution of sys_tas(). In this case, the following write operation ("*addr = 1;") overwrites the __pthread_unlock() result, and sys_tas() fails to get a lock in the next turn and after that. According to the attatched patch, sys_tas() returns 0 value in the next turn and deadlocks never happen. Signed-off-by: Hitoshi Yamamoto Signed-off-by: Hirokazu Takata Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- arch/m32r/kernel/sys_m32r.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/m32r/kernel/sys_m32r.c b/arch/m32r/kernel/sys_m32r.c index e0500e1..fe55b28 100644 --- a/arch/m32r/kernel/sys_m32r.c +++ b/arch/m32r/kernel/sys_m32r.c @@ -41,7 +41,8 @@ asmlinkage int sys_tas(int *addr) return -EFAULT; local_irq_save(flags); oldval = *addr; - *addr = 1; + if (!oldval) + *addr = 1; local_irq_restore(flags); return oldval; } @@ -59,7 +60,8 @@ asmlinkage int sys_tas(int *addr) _raw_spin_lock(&tas_lock); oldval = *addr; - *addr = 1; + if (!oldval) + *addr = 1; _raw_spin_unlock(&tas_lock); return oldval; -- 1.8.2.3