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

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Date:	Thu, 30 Apr 2015 11:46:59 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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

On Wed, 2015-04-29 at 21:44 +0200, Manfred Spraul wrote:
> 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.

Thanks for confirming.

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

Agreed.

> 
> 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.

No, this is not the case. All users are expected to do the wakeup right
away.

Thanks,
Davidlohr

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