linux-kernel - Re: [PATCH] firmware loader: don't cancel _nowait requests when helper is not yet available

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Message-Id: <201203150013.45749.rjw@sisk.pl>
Date:	Thu, 15 Mar 2012 00:13:45 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	Kay Sievers <kay.sievers@...y.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Christian Lamparter <chunkeey@...glemail.com>,
	linux-kernel@...r.kernel.org,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	alan@...rguk.ukuu.org.uk,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux PM mailing list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] firmware loader: don't cancel _nowait requests when helper is not yet available

On Thursday, March 15, 2012, Rafael J. Wysocki wrote:
> On Wednesday, March 14, 2012, Stephen Boyd wrote:
> > On 03/13/12 13:14, Rafael J. Wysocki wrote:
> > > All of those use cases are in fact of the "wait for user space to be thawed
> > > and then load the firmware" type, which I believe may be handled without
> > > changing that code.
> > >
> > > Why don't you make your kthread freezable, for one example?
> > >
> > > Why don't you use a freezable workqueue instead?
> > >
> > 
> > If we put it on the freezable workqueue or make it a freezable thread
> > will it work?
> 
> That depends on what exactly you want to achieve, which isn't entirely clear
> to me at this point.
> 
> > In my scenario a wakeup interrupt comes in that wakes us up from
> > suspend. Within that wakeup handler a work item is scheduled to the
> > freezable workqueue. That work item then calls request_firmware().
> 
> That should work.
> 
> > It looks like we call schedule() after thawing the workqueues and tasks
> > so the work item could run before usermodehelpers are enabled and then
> > request_firmware() would fail. Do we need something like this (ignore
> > the fact that we call usermodhelper_enable() twice)?
> > 
> > diff --git a/kernel/power/process.c b/kernel/power/process.c
> > index 7e42645..61bfa95 100644
> > --- a/kernel/power/process.c
> > +++ b/kernel/power/process.c
> > @@ -187,6 +187,7 @@ void thaw_processes(void)
> >         } while_each_thread(g, p);
> >         read_unlock(&tasklist_lock);
> >  
> > +       usermodehelper_enable();
> 
> That would be a reasonable change.
> 
> >         schedule();
> >         printk("done.\n");
> >  }
> > 
> > 
> > Is there a reason we disable usermodehelpers if
> > CONFIG_SUSPEND_FREEZER=n?
> 
> Not really, but CONFIG_SUSPEND_FREEZER=n can only work reliably in a
> very limited set of cases, so I don't think it's even worth making the
> general code depend on it.
> 
> I'd actually prefer to remove CONFIG_SUSPEND_FREEZER altogether,
> because it's not very useful nowadays (probably isn't useful at all).
> 
> > Should we do this instead so that
> > usermodehelpers are only disabled if we freeze userspace? Also what is
> > that schedule() call in thaw_kernel_threads() for? It looks like we'll
> > call schedule between kernel thread thawing and userspace thawing.
> 
> Which is OK, I think.

Moreover, thaw_kernel_threads() is _only_ called by (a) freeze_kernel_threads()
on error and (b) user-space hibernate interface in kernel/power/user.c
(and please read the comment in there describing what it's there for, which
also explains why the schedule() call in there is necessary).

In all other cases, thaw_processes() is used to thaw kernel threads and user
space at the same time, so I believe we don't need the changes you're proposing
in that area.

Thanks,
Rafael
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