Re: [PATCH v2] ipc/msg: Implement lockless pipelined wakeups

From: Davidlohr Bueso
Date: Wed Nov 04 2015 - 13:11:52 EST

On Tue, 03 Nov 2015, Sebastian Andrzej Siewior wrote:

In pipelined_receive() (mqueue) there is the wake_q_add() with the
implicit cmpxchg barrier. The matching barrier pairing should be in
wq_sleep() but there is none. Why is it okay to have none there and I
need one here?

So the pairing in mqueue is done in wq_sleep() as we unlock info->lock,
so there's an implied barrier there.

Also, considering Peter's suggestion to wrap around READ/WRITE_ONCE
for sysv queues (specially since you got rid of the volatile tag),
it seems we also need them for mqueues.

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