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 01:11:05 +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 16:34, Rafael J. Wysocki wrote:
> > 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:
> >>>
> >>>> 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.
> >
> 
> Yes but if someone calls request_firmware() from a kthread then they
> will hit the same problem where the thread runs and requests the
> firmware and usermodehelpers are still disabled. Currently my code is
> written with kthreads and that thread makes the request firmware call,
> so this doesn't seem far fetched (although in my case I can probably fix
> it).

So again, please consider using suspend/resume notifiers to synchronize
your kthread with system power transitions.

> It looks like before 379e0be (PM / Freezer: Thaw only kernel
> threads if freezing of kernel threads fails, 2012-02-03) schedule wasn't
> called until userspace was thawed. It looks like that patch was about
> hibernation and not suspend?

It was about hibernation and it fixed a real bug.

Thanks,
Rafael


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