[PATCH] ARM: spinlock: avoid exclusive accesses on unlock()path

On Wed, Aug 08, 2018 at 08:30:28AM +0800, ??? wrote:

On 8 Aug 2018, will.deacon wrote:

quoted

On Tue, Aug 07, 2018 at 09:59:13AM +0800, ??? wrote:

quoted

while one cpu is doing arch_spin_unlock,another cpu doing arch_spin_lock
the first cpu's
lock->tickets.owner++;
may be over writed by the second's
strex %2, %1, [%3]\n
so the owner filed doesn't get increased.

quoted

I don't see how this can happen. If the unlocker writes to the owner field
in-between the locker's LDXR/STXR pair (which is the only way the harmful
overwrite could occur as far as I can tell), then the STXR will fail and not
update memory.

quoted

Are you actually seeing a problem here?

In DDI0406C_C_arm_architecture_reference_manual.pdf A3-117, It says after
normal Store operation,  the monitor maybe in Exclusive Access or Open Access
which is implementation defined.

You're looking at the section describing non-shareable memory. If you scroll
down to A3-119, you can see that a store to the tagged address from a
different CPU will transition the monitor to the open state.

Will