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

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Date:	Thu, 15 Mar 2012 00:34:08 +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, Stephen Boyd wrote:
> On 03/14/12 16:13, Rafael J. Wysocki wrote:
> > 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).
> 
> Exactly. So in case (a) when the error occurs we'll have this call flow:
> 
> usermodehelpers_disable()
> suspend_freeze_processes()
>     freeze_processes()
>     freeze_kernel_threads()
>         try_to_freeze_tasks() <-- returns error
>         thaw_kernel_threads()
>             schedule()
>     thaw_processes()
> usermodehelpers_enable()
> 
> Shouldn't we schedule only after we thaw all processes (not just tasks)?
> Otherwise we may run a kernel thread before userspace is thawed?

Yes, we may, but that isn't wrong, is it?

Only a few kernel threads are freezable, so definitely kernel threads
can run while user space is frozen.

Thanks,
Rafael
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