linux-kernel - Re: [RFC] firmware loader: retry _nowait requests when userhelper is not yet available

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Message-Id: <201203170123.10599.rjw@sisk.pl>
Date:	Sat, 17 Mar 2012 01:23:10 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christian Lamparter <chunkeey@...glemail.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
	alan@...rguk.ukuu.org.uk,
	Linux PM mailing list <linux-pm@...r.kernel.org>
Subject: Re: [RFC] firmware loader: retry _nowait requests when userhelper is not yet available

On Saturday, March 17, 2012, Linus Torvalds wrote:
> On Fri, Mar 16, 2012 at 3:57 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > +       if (nowait) {
> > +               int limit = loading_timeout * MSEC_PER_SEC;
> > +               int timeout = 10;  /* in msec */
> > +
> > +               while (usermodehelper_is_disabled()) {
> > +                       read_unlock_usermodehelper();
> > +
> > +                       msleep(timeout);
> 
> Ugh, this is disgusting.
> 
> The whole point of nowait was that it's not synchronous - so it should
> just be driven by timers, not some kind of random "while sleep" loop.
> 
> And that fw thing already does have a timeout associated with it, and
> quite frankly, the *sane* approach is to do all this not in
> _request_firmware(), but in request_firmware_work_func() - never even
> call _request_firmware() in the first place if the system isn't ready,
> just reset the timeout to retry it again later.
> 
> Seriously. The rule should be really simple: nothing should *ever*
> call "request_firmware()" (or the _request_firmware() helper function)
> while the system is not up. That WARN_ON() should remain totally and
> utterly unconditional, and it should *not* be conditional on "nowait"
> or any idiotic crappy hack like that.
> 
> If there is an asynchronous thread - and there is, for the _nowait()
> case - that asynchronous thread should set up the timer and retry in
> ten seconds or whatever. It should *not* call 'request_firmware()" and
> expect that to do something special.

OK, but that asynchronous thread needs to know whether or not the system is up.

It can use the usermodehelper_is_disabled() check, but that needs to be done
under read_lock_usermodehelper() and it can't release the lock before
calling _request_firmware(), or all that thing would be racy.  If it doesn't
release the lock, then _request_firmware() will take it again, reentrantly,
which isn't nice.  So, it looks like it has to use a different check.

I was pondering a suspend/resume notifier that would block either
request_firmware_work_func() (possibly with a timeout), if system suspend is
in progress, or system suspend, if _request_firmware() is in progress,
but that wouldn't cover the initialization case.

Thanks,
Rafael
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