linux-kernel - Re: [PATCH v2] ipc/mqueue: remove STATE

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Date:	Wed, 29 Apr 2015 21:44:28 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Darren Hart <darren@...art.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	fredrik.markstrom@...driver.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] ipc/mqueue: remove STATE_PENDING

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
  aaaa
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
  bbbb
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.

--
     Manfred
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