Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 26 Jun 2018 12:11:28 +0200
From:   Andrea Parri <andrea.parri@amarulasolutions.com>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        Will Deacon <will.deacon@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Ingo Molnar <mingo@redhat.com>,
        Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCH] doc: Update wake_up() & co. memory-barrier guarantees
Message-ID: <20180626101127.GB8295@andrea>
References: <1529918258-7295-1-git-send-email-andrea.parri@amarulasolutions.com>
 <Pine.LNX.4.44L0.1806251253380.1779-100000@iolanthe.rowland.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Pine.LNX.4.44L0.1806251253380.1779-100000@iolanthe.rowland.org>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

> > -A write memory barrier is implied by wake_up() and co.  if and only if they
> > -wake something up.  The barrier occurs before the task state is cleared, and so
> > -sits between the STORE to indicate the event and the STORE to set TASK_RUNNING:
> > +A general memory barrier is executed by wake_up() if it wakes something up.
> > +If it doesn't wake anything up then a memory barrier may or may not be
> > +executed; you must not rely on it.  The barrier occurs before the task state
> > +is accessed, in part., it sits between the STORE to indicate the event and
> > +the STORE to set TASK_RUNNING:
> 
> Minor suggestion: Instead of "in part.", how about "that is"?
> 
> (I generally find "in part." to be at least a little confusing,
> probably because "part" is itself a word and "in part" is a 
> reasonably common phrase in English.)

Mmh, the fact is that that "before the task state is accessed" does want
to include the LOAD from ->state to check for the task state (recall the
pattern in [1])...; how about if I expand "in part." to "in particular"?


> 
> >  
> > -	CPU 1				CPU 2
> > +	CPU 1 (Sleeper)			CPU 2 (Waker)
> >  	===============================	===============================
> >  	set_current_state();		STORE event_indicated
> >  	  smp_store_mb();		wake_up();
> > -	    STORE current->state	  <write barrier>
> > -	    <general barrier>		  STORE current->state
> > -	LOAD event_indicated
> > +	    STORE current->state	  ...
> > +	    <general barrier>		  <general barrier>
> > +	LOAD event_indicated		  if ((LOAD task->state) & TASK_NORMAL)
> > +					    STORE task->state
> >  
> > -To repeat, this write memory barrier is present if and only if something
> > -is actually awakened.  To see this, consider the following sequence of
> > -events, where X and Y are both initially zero:
> > +where "task" is the thread being woken up and it equals CPU 1's current.
> 
> Since "task" is in quotation marks, "current" should also be in
> quotation marks.

Sure, will fix in v2.

Thanks,
  Andrea


> 
> Alan
>