linux-kernel - Re: btusb "firmware request while host is not available" at resume

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Message-ID: <20170911200651.GB16216@wotan.suse.de>
Date:   Mon, 11 Sep 2017 22:06:51 +0200
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Marcel Holtmann <marcel@...tmann.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Gabriel C <nix.or.die@...il.com>, Tejun Heo <tj@...nel.org>,
        "Gustavo F. Padovan" <gustavo@...ovan.org>,
        Sukumar Ghorai <sukumar.ghorai@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "bluez mailin list (linux-bluetooth@...r.kernel.org)" 
        <linux-bluetooth@...r.kernel.org>
Subject: Re: btusb "firmware request while host is not available" at resume

On Mon, Sep 11, 2017 at 12:29:55PM -0700, Greg Kroah-Hartman wrote:
> On Mon, Sep 11, 2017 at 07:11:38PM +0200, Luis R. Rodriguez wrote:
> > On Mon, Sep 11, 2017 at 06:46:47AM -0700, Greg Kroah-Hartman wrote:
> > > To confirm, reverting this fixes the problem I was seeing in 4.13.  I've
> > > queued it up for the next 4.13-stable release as well.
> > 
> > Commit 81f95076281f ("firmware: add sanity check on shutdown/suspend") may
> > seem kludgy but the reason for it was to cleanup the horrible forced and
> > required UMH lock even when the UMH lock was *not* even needed, which was later
> > removed via commit 06a45a93e7d34aa ("firmware: move umh try locks into the umh
> > code").
> 
> So what does this mean now that it is reverted?

We discuss what we should do about upkeeping a warning in the future, as
I think technically the warning was still valid and it could help avoid
racy lookups with the filesystem which otherwise could have gone unnoticed.

> > Removing the old UMH lock even when the UMH lock was *not* needed was the right
> > thing to do but commit 81f95076281f (("firmware: add sanity check on
> > shutdown/suspend") was put in place as a safe guard as the lock was also
> > placing an implicit sanity check on the API. It ensures the API with the cache
> > was used as designed, otherwise you do run the risk of *not getting the
> > firmware you may need* -- Marcel seems to acknowledge this possibility.
> > 
> > It may be possible for us to already have in place safeguards so that upon
> > resume we are ensuring the path to the firmware *is* available, so IMHO we
> > should remove this *iff* we can provide this guarantee.  Otherwise the check is
> > valid. You see, although the UMH lock was bogus, it did implicitly ask the
> > question: is it safe for *any* helper to run and make assumptions on the
> > filesystem then?
> > 
> > In lieu of this question being answered the warning is valid given the design
> > of the firmware API and the having the cache available as a measure to resolve
> > this race.
> 
> I don't understand what you are trying to say here at all.
> 
> To be specific, what, if anything, is a problem with the current state
> of Linus's tree (and the next 4.13-stable release)?

The warning is issued when drivers issue *new* firmware requests on resume. The
firmware API cache was designed to enable drivers to easily be able to request
firmware on resume without concern about races against the filesystem, but in order
for this to work the drivers must have requested the firmware prior to suspend.

> If something needs to be fixed, can you make a patch showing that?  Or
> do we also need to revert anything else as well to get back to a "better
> working" state?

I took a look at the driver and it seems that btusb_setup_intel_new() is
not called after the driver is initialized, rather its called only when
hci_dev_do_open() is called. Its not clear to me how you can end up calling
this on resume but not prior to this on a running system. Feedback from
someone more familiar with bt would be useful.

I'd have the call for firmware on probe, no processing would be needed, just
a load to kick the cache into effect would suffice. This may require a bit
of code shift so its best someone more familiar do this.

If it confirmed this information is helping avoid these races we can reconsider
re-instating the warn as a firmware dev debugging aid for developers.

If the race this warning complained about is indeed possible the same race is
also possible for other usermode helpers. Its *why* the UMH lock was
implemented, it however was never generalized.

  Luis