[PATCH] m32r: Fix sys_tas() syscall
[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 <14a8518c98ba6ba12ef8864bc763ee959a363006@ap.MitsubishiElectric.co.jp>
Signed-off-by: Hirokazu Takata <cf11bbe1b6d3d076b415c503f9127c50075c83bd@linux-m32r.org>
Signed-off-by: Andrew Morton <5c1e68b099950c134891f0b6e179498a8ebe9cf9@osdl.org>
Signed-off-by: Linus Torvalds <69652caca27c8b940640ad396ab71f93cacec34f@osdl.org>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs