linux-kernel - Re: [PATCH v2] firmware: fix sending -ERESTARTSYS due to signal on fallback

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Date:   Thu, 8 Jun 2017 18:10:20 -0700
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Martin Fuzzey <mfuzzey@...keon.com>
Cc:     linux-fsdevel@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
        "Ted Ts'o" <tytso@....edu>, Andy Lutomirski <luto@...nel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        Linux API <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Daniel Wagner <wagi@...om.org>,
        David Woodhouse <dwmw2@...radead.org>,
        jewalt@...innovations.com, rafal@...ecki.pl,
        Arend Van Spriel <arend.vanspriel@...adcom.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Li, Yi" <yi1.li@...ux.intel.com>, atull@...nsource.altera.com,
        Moritz Fischer <moritz.fischer@...us.com>,
        Petr Mladek <pmladek@...e.com>,
        Johannes Berg <johannes.berg@...el.com>,
        Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        Luca Coelho <luciano.coelho@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        David Howells <dhowells@...hat.com>,
        Peter Jones <pjones@...hat.com>,
        Hans de Goede <hdegoede@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] firmware: fix sending -ERESTARTSYS due to signal on fallback

On Wed, Jun 7, 2017 at 10:54 AM, Martin Fuzzey <mfuzzey@...keon.com> wrote:
> On 07/06/17 19:08, Luis R. Rodriguez wrote:
>>
>> On Thu, May 25, 2017 at 10:28:38AM +0200, Fuzzey, Martin wrote:
>>>
>>> 1) Android init calls write() on the sysfs file
>>> 2) The sysfs .store() callback registered by a driver is called
>>> 3) The driver calls request_firmware()
>>> 4) request_firmware() sends the firmware load request to userspace and
>>> calls wait_for_completion_interruptible()
>>
>> Martin, just for completeness on documenting on the commit log of the next
>> swait proposed fix for this -- what signal did the process get from which
>> you
>> note the child dies below ? Exactly what in Android sent this signal ?
>
>
> Android didn't send the signal, the kernel did (SIGCHLD).
>
> Like this:
>
> 1) Android init (pid=1) fork()s (say pid=42) [this child process is totally
> unrelated to firmware loading]
> 2) Android init (pid=1) does a write() on a (driver custom) sysfs file which
> ends up calling request_firmware() kernel side
> 3) The firmware loading fallback mechanism is used, the request is sent to
> userspace and pid 1 waits in the kernel on wait_*
> 4) before firmware loading completes pid 42 dies (for any reason - in my
> case normal termination)

Interesting, could one interpretation here be that the process
successfully finishing + the signal being sent beats out the timing of
the firmware_class syfs code detecting that the write completed ?

> 5) Kernel delivers SIGCHLD to pid=1 to tell it a child has died, which
> causes -ERESTARTSYS to be returned from wait_*

  Luis