Messages in this thread Patch in this message | | | Date | Fri, 25 Nov 2005 10:57:46 +0900 (JST) | Subject | [PATCH 2.6.15-rc2] m32r: Fix sys_tas() syscall | From | Hirokazu Takata <> |
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Hello,
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 <Yamamoto.Hitoshi@ap.MitsubishiElectric.co.jp> Signed-off-by: Hirokazu Takata <takata@linux-m32r.org> ---
arch/m32r/kernel/sys_m32r.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
Index: linux-2.6.14-mm1/arch/m32r/kernel/sys_m32r.c =================================================================== --- linux-2.6.14-mm1.orig/arch/m32r/kernel/sys_m32r.c 2005-11-07 19:31:23.404878304 +0900 +++ linux-2.6.14-mm1/arch/m32r/kernel/sys_m32r.c 2005-11-07 19:33:04.991434800 +0900 @@ -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; -- Hirokazu Takata <takata@linux-m32r.org> Linux/M32R Project: http://www.linux-m32r.org/ - 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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