linux-kernel - Re: [PATCH] doc: Update wake_up() & co. memory-barrier guarantees

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Date:   Tue, 26 Jun 2018 12:11:28 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] doc: Update wake_up() & co. memory-barrier guarantees

> > -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
>