linux-kernel - Re: complete_all and "forever" completions

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Message-ID: <20161026084535.GX3102@twins.programming.kicks-ass.net>
Date:   Wed, 26 Oct 2016 10:45:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        computersforpeace@...il.com, Ingo Molnar <mingo@...nel.org>,
        der.herr@...r.at
Subject: Re: complete_all and "forever" completions

On Tue, Oct 25, 2016 at 03:30:54PM -0700, Dmitry Torokhov wrote:
> Hi,
> 
> Reading Documentation/scheduler/completion.txt, complete_all() is

Oh, there is documentation? /me goes read.

> supposed to be usable with "forever" completions, i.e. when we have an
> action that happens once and stays "done" for the rest of lifetime of an
> object, no matter how many times we check for "doneness".

I suppose you allude to this wording:

  "calls complete_all() to signal all current and future waiters."

> However the
> implementation for complete_all() simply sets the counter to be greater
> or equal UINT_MAX/2 and do_wait_for_common() happily decreases it on
> every call.

This is indeed so.

> Is it simply an artefact of [older] implementation where we do not
> expect to make that many calls to wait_for_completion*() so that
> completion that is signalled with ocmplete_all() is practically stays
> signalled forever?

The text says it was written against v3.18 or thereabout, and that
implementation looks a lot like todays, so I doubt it ever worked like
that.

> Or do we need something like this in
> do_wait_for_common():
> 
> 	if (x->done < UINT_MAX/2)
> 		x->done--;

Depends a bit, do you really want this? Seems a bit daft to keep asking
if its done already, seems like a waste of cycles to me.