[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: NHitoshi Yamamoto <Yamamoto.Hitoshi@ap.MitsubishiElectric.co.jp>
Signed-off-by: NHirokazu Takata <takata@linux-m32r.org>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

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;