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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>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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