Re: [PATCH/RFC] wait_for_completion_timeout() considered harmful.

[NeilBrown <neilb@suse.de>]

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On Mon, 18 Nov 2013 15:27:46 -0800 Andrew Morton <akpm@linux-foundation.org>
wrote:

> On Sun, 17 Nov 2013 08:06:03 +1100 NeilBrown <neilb@suse.de> wrote:
> 
> > It would be reasonable to assume that
> > 
> >       wait_for_completion_timeout(&wm8350->auxadc_done, msecs_to_jiffies(5));
> > 
> > would wait at least 5 msecs for the auxadc_done to complete.  But it does not.
> > With a HZ of 200 or less, msecs_to_jiffies(5) has value '1', and so this
> > will only wait until the next "timer tick", which could happen immediately.
> > 
> > This can lead to incorrect results - and has done so in out-of-tree patches
> > for drivers/misc/bmp085.c which uses a very similar construct to enable interrupt
> > based result collection.
> > 
> > The documentation for several *_timeout* functions claim they will wait for
> > "timeout jiffies" to have elapsed where this is not the case.  They will
> > actually wait for "timeout" jiffies to have started implying an elapsed time
> > between (timeout-1) and (timeout).
> > 
> > This patch corrects some of this documentation, and adds a collection of
> >   wait_for_completion*_msecs()
> > interfaces which wait at least the given number of milliseconds for the
> > completion (or a signal).
> 
> Mutter.  wait_for_x(..., 5ms) should wait for a minimum of 5ms, no matter
> what.
> 
> So I'd suggest we make that happen, rather than adding some new interfaces?

I thought of that.  It would certainly be nice.

However what we have is 
         XXX_timeout(...., jiffies).
And if we decided that
         XXX_timeout(...., msecs_to_jiffies(5))
would only timeout after at least 5ms, then
         schedule_timeout(1)
would have to wait at least one full jiffie, which is quite different to what
it currently does.

We have loops that have
    timeout = schedule_timeout(timeout)
in the middle and if we change the semantics of schedule_timeout() to round
up, those loops could wait quite a bit longer than expected.

So I think that we do need to add new interfaces just like msleep() was introduced
a while back to fix all the various misuses of
    schedule_timeout(msecs_to_jiffies(XX))).

Possibly we can also discard old bad interfaces.
Maybe the *_timeout() interfaces should become *_until() where the jiffies
number isn't a count but is a value that we wait for "jiffies" to exceed.

I don't think there is a really easy solution, but thanks for pushing the
discussion along towards trying to understand one.

NeilBrown


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