Re: [PATCH v2] ipc/mqueue: remove STATE_PENDING

From: Manfred Spraul
Date: Wed Apr 29 2015 - 15:44:38 EST

Hi Davidlohr,

On 04/28/2015 06:59 PM, Davidlohr Bueso wrote:
On Tue, 2015-04-28 at 18:43 +0200, Peter Zijlstra wrote:
Well, if you can 'guarantee' the cmpxchg will not fail, you can then
rely on the fact that cmpxchg implies a full barrier, which would
obviate the need for the wmb.
Yes, assuming it implies barriers on both sides. And we could obviously
remove the need for pairing. With wake_q being local to wq_sleep() I
cannot see duplicate tasks trying to add themselves in the list. Failed
cmpxchg should only occur when users start misusing the wake_q.

Manfred, do you have any objections to this? Perhaps I've missed the
real purpose of the barriers.
I don't remember the details either, so let's check what should happen:

CPU1: sender copies message to kernel memory
CPU1: sender does receiver->msg = message;
** barrier 1
CPU1: sender does receiver->state = STATE_READY;

CPU2: receiver notices receiver->state = STATE_READY;
** barrier 2
CPU2: receiver reads receiver->msg
CPU2: receiver reads *receiver->msg

Failures would be:
- write to receiver->state is visible before the write to receiver->msg or to *receiver->msg
** barrier 1 needs to be an smp_wmb()
- cpu 2 reads receiver->msg before receiver->state
** barrier 2 needs to be an smp_rmb().

As far as I can see, no barrier is needed in pos aaaa or bbbb.

With regards to failed cmpxchg():
I don't see that mqueue could cause it by itself.

Who is allowed to use wake_q?
If it is permitted to use wake_q for e.g. timeout/signal delivery wakeup, then that user might have a pending wakeup stored in the task struct.

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