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

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Date:   Wed, 26 Oct 2016 09:15:19 +0000
From:   Nicholas Mc Guire <der.herr@...r.at>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        computersforpeace@...il.com, Ingo Molnar <mingo@...nel.org>
Subject: Re: complete_all and "forever" completions

On Wed, Oct 26, 2016 at 10:45:35AM +0200, Peter Zijlstra wrote:
> 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.

bad wording maybe - the intent of setting it to UINT_MAX/2
as far as I can judge is though that UINT_MAX/2 effectively would be
infinity in practice. Is it realistic to assume that there would be
a complete_all() call followed by 2147483648 calls to wait_for_completion() ?
The note on "future waiters" was to make it clear that once you called
complete_all() future wait_for_completion() have no synchronizing effect.

> 
> > 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.
> 

I would claim that if you have a complete_all() (done=2147483648) and you
actually did manage to decrement it to 0 over time so a call finally blocks
(presumably for ever) this would be uncovering a deisgn bug in the use of
completion as such a setup does not make any sense (Or Im just not creative
enough to think of such a situation).

thx!
hofrat