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From: "Luis R. Rodriguez" <mcgrof@kernel.org>
Date: Thu, 8 Jun 2017 18:10:20 -0700
Message-ID: <CAB=NE6WHoP21BJTb7jmNVZJruHr9zWCTOyDtyhgQWy7CT=6q0g@mail.gmail.com>
Subject: Re: [PATCH v2] firmware: fix sending -ERESTARTSYS due to signal on fallback
To: Martin Fuzzey <mfuzzey@parkeon.com>
Cc: linux-fsdevel@vger.kernel.org, Alan Cox <alan@linux.intel.com>,
        "Ted Ts'o" <tytso@mit.edu>, Andy Lutomirski <luto@kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
        Linux API <linux-api@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
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On Wed, Jun 7, 2017 at 10:54 AM, Martin Fuzzey <mfuzzey@parkeon.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