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

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Date:   Mon, 25 Jun 2018 12:56:14 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
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

On Mon, 25 Jun 2018, Andrea Parri wrote:

> Both the implementation and the users' expectation [1] for the various
> wakeup primitives have evolved over time, but the documentation has not
> kept up with these changes: brings it into 2018.
> 
> [1] http://lkml.kernel.org/r/20180424091510.GB4064@hirez.programming.kicks-ass.net
> 
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
> [ aparri: Apply feedback from Alan Stern. ]
> Cc: Alan Stern <stern@...land.harvard.edu>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Cc: David Howells <dhowells@...hat.com>
> Cc: Jade Alglave <j.alglave@....ac.uk>
> Cc: Luc Maranget <luc.maranget@...ia.fr>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Cc: Akira Yokosawa <akiyks@...il.com>
> Cc: Daniel Lustig <dlustig@...dia.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> ---
>  Documentation/memory-barriers.txt | 43 ++++++++++++++++++++++++---------------
>  kernel/sched/completion.c         |  8 ++++----
>  kernel/sched/core.c               | 11 +++++-----
>  kernel/sched/wait.c               | 24 ++++++++++------------
>  4 files changed, 47 insertions(+), 39 deletions(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index a02d6bbfc9d0a..bf58fa1671b62 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -2179,32 +2179,41 @@ or:
>  	event_indicated = 1;
>  	wake_up_process(event_daemon);
>  
> -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.)

>  
> -	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.

Alan