Message-Id: <201203142354.04256.rjw@sisk.pl>
Date:	Wed, 14 Mar 2012 23:54:04 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Kay Sievers <kay@...y.org>, Greg KH <gregkh@...uxfoundation.org>,
	Christian Lamparter <chunkeey@...glemail.com>,
	linux-kernel@...r.kernel.org, alan@...rguk.ukuu.org.uk,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux PM mailing list <linux-pm@...r.kernel.org>,
	skannan@...eaurora.org, Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH] firmware loader: don't cancel _nowait requests when helper is not yet available

On Wednesday, March 14, 2012, Srivatsa S. Bhat wrote:
> On 03/14/2012 05:40 AM, Rafael J. Wysocki wrote:
> 
> > On Wednesday, March 14, 2012, Kay Sievers wrote:
> >> On Tue, Mar 13, 2012 at 20:42, Rafael J. Wysocki <rjw@...k.pl> wrote:
> >>> On Sunday, March 11, 2012, Kay Sievers wrote:
> >>>> On Sat, Mar 10, 2012 at 00:36, Greg KH <gregkh@...uxfoundation.org> wrote:
> >>
> >>>>> What does uevent have to do with things here?
> >>>>
> >>>> I don't think that the firmware loader should care about the
> >>>> usermodehelper at all, and that stuff fiddling should just be removed
> >>>> from the firmware class.
> >>>
> >>> It's there to warn people that their drivers do stupid things like
> >>> loading frimware during system resume, which is guaranteed not to work.
> >>>
> >>> IOW, it's there very much on purpose.
> >>
> >> Using the /sbin/hotplug is no case that needs any warning. It' such a
> >> broken model these days, that firmware loading is the least problem
> >> that occurs with it.
> >>
> >>>> Forking /sbin/hotplug is disabled by default, it is a broken concept,
> >>>> and it cannot work reliably on today's systems.
> >>>>
> >>>> Firmware is not loaded by /sbin/hotplug since many years, but by udev
> >>>> or whatever service handles uevents, like ueventd on android.
> >>>
> >>> Which I'm not sure why is relevant here.
> >>
> >> It is relevant in the sense that the firmware loader should not even
> >> know that a uevent *can* cause a usermodehelper exec() if it runs in
> >> legacy mode. The firmware loader just has no business in fiddling with
> >> the details of driver core legacy stuff. I don't think his warning
> >> makes much sense.
> > 
> > But that warning actually triggers for drivers that attempt to use
> > request_firmware() during system resume, even though /sbin/hotplug isn't
> > used any more.
> > 
> 
> 
> I agree with Rafael about why the warning and the bail out is required,
> including the part about the races with freezer which he explained in his
> other mail. These problems have already been well documented too.
> (See Documentation/power/freezing-of-tasks.txt).
> 
> > usermodehelper_is_disabled() means "we are in the middle of system power
> > transition" rather than anything else (I agree it should be called
> > suspend_in_progress() or something similar these days).
> > 
> 
> 
> How about this patch then?
> 
> ---
> 
> From: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> Subject: PM/firmware loader: Use better name for usermodehelper_is_disabled()
> 
> Rafael J. Wysocki <rjw@...k.pl> wrote:
> 
> | usermodehelper_is_disabled() means "we are in the middle of system power
> | transition" rather than anything else (I agree it should be called
> | suspend_in_progress() or something similar these days).
> 
> 
> But simply renaming usermodehelper_is_disabled() to suspend_in_progress()
> isn't the best thing to do since that would be misleading because suspend
> transitions are begun much before usermodehelpers are disabled.
> 
> Apart from that, we don't want people to suddenly start abusing this function
> in future in a totally different context to check if suspend is in progress.
> 
> So, add an alias specific to firmware loaders alone, that will internally
> call usermodehelpers_is_disabled().
> 
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> ---
> 
>  drivers/base/firmware_class.c |   12 +++++++++++-
>  1 files changed, 11 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/base/firmware_class.c b/drivers/base/firmware_class.c
> index 6c9387d..9e401e1 100644
> --- a/drivers/base/firmware_class.c
> +++ b/drivers/base/firmware_class.c
> @@ -510,6 +510,8 @@ static void fw_destroy_instance(struct firmware_priv *fw_priv)
>  	device_unregister(f_dev);
>  }
>  
> +#define suspend_in_progress()	usermodehelper_is_disabled()

This looks like an overstretch to me.  I think a comment would be sufficient.

> +
>  static int _request_firmware(const struct firmware **firmware_p,
>  			     const char *name, struct device *device,
>  			     bool uevent, bool nowait)
> @@ -535,7 +537,15 @@ static int _request_firmware(const struct firmware **firmware_p,
>  
>  	read_lock_usermodehelper();
>  
> -	if (WARN_ON(usermodehelper_is_disabled())) {
> +	/*
> +	 * It is wrong to request firmware when the system is suspended,
> +	 * because it simply won't work reliably.

In fact, it won't work at all.

> +	 Also, it can cause races with
> +	 * the freezer, leading to freezing failures.

It actually is worse than that too.  It may cause a user space process
to run when we think we have frozen user space and _that_ may lead to
all kinds of interesting breakage.

> 	 * So check if the system is
> +	 * in a state which is unsuitable for requesting firmware (because the
> +	 * system is suspended or not yet fully resumed) and bail out early if
> +	 * needed.

And here I'd explain why usermodehelper_is_disabled() is used for that.

> +	 */
> +	if (WARN_ON(suspend_in_progress())) {
>  		dev_err(device, "firmware: %s will not be loaded\n", name);
>  		retval = -EBUSY;
>  		goto out;

Thanks,
Rafael
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