Commit 91f4ab05 authored by Hirokazu Takata's avatar Hirokazu Takata Committed by Linus Torvalds

[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: default avatarHitoshi Yamamoto <Yamamoto.Hitoshi@ap.MitsubishiElectric.co.jp>
Signed-off-by: default avatarHirokazu Takata <takata@linux-m32r.org>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent bce61dd4
......@@ -41,6 +41,7 @@ asmlinkage int sys_tas(int *addr)
return -EFAULT;
local_irq_save(flags);
oldval = *addr;
if (!oldval)
*addr = 1;
local_irq_restore(flags);
return oldval;
......@@ -59,6 +60,7 @@ asmlinkage int sys_tas(int *addr)
_raw_spin_lock(&tas_lock);
oldval = *addr;
if (!oldval)
*addr = 1;
_raw_spin_unlock(&tas_lock);
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment