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

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Date:   Mon, 11 Sep 2017 17:33:57 -0700
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Gabriel C <nix.or.die@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Marcel Holtmann <marcel@...tmann.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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 5:13 PM, Gabriel C <nix.or.die@...il.com> wrote:
> On 11.09.2017 22:06, Luis R. Rodriguez wrote:
>>
>> 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.
>
>
> While I really don't know that code I can tell anything about .. however
> this is _NOT_
> about Intel Hardware only .. I trigger that on laptops with Atheros..

If the Atheros driver works in the same way then the same race exists
there as well.

  Luis