Subject: Re: [PATCH 0/4] firmware: fix fallback mechanism by ignoring SIGCHLD
To: "Luis R. Rodriguez" <mcgrof@kernel.org>, gregkh@linuxfoundation.org
References: <20170614222017.14653-1-mcgrof@kernel.org>
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From: Martin Fuzzey <mfuzzey@parkeon.com>
Message-ID: <59423BFE.3060405@parkeon.com>
Date: Thu, 15 Jun 2017 09:49:18 +0200
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On 15/06/17 00:20, Luis R. Rodriguez wrote:
> Martin reported an issue with Android where if sysfs is used to trigger a sync
> fw load which *relies* on the fallback mechanism and a background job completes
> while the trigger is ongoing in the foreground it will immediately fail the fw
> request.  The issue can be observed in this simple test script using the
> test_firmware driver:
>
> 	set -e
> 	/etc/init.d/udev stop
> 	modprobe test_firmware
> 	DIR=/sys/devices/virtual/misc/test_firmware
> 	echo 10 >/sys/class/firmware/timeout
> 	sleep 2 &
> 	echo -n "does-not-exist-file.bin" > "$DIR"/trigger_request
>
> The background sleep triggers the SIGCHLD signal and we fail the firmware
> request on the fallback mechanism. This was due to the type of wait used which
> ...
>
> Note that although I *feared* this might implicate any use of non-killable waits
> on other system calls, such as finit_module(), initial testing confirms this to
> not be the case.  For instance replacing the echo with modprobe on a module
> which does the same on init does not present the same issues. This could be due
> to the special SA_RESTART flag case on write() as noted above and sysfs...
> however, its not perfectly clear yet to me.
>
>
The reason the problem does not occur with modprobe is that in that case 
the processes triggering the firmware load
(modprobe) and the process dying (sleep) are *siblings* rather than 
father and child.
So the modprobe process does *not* receive a SIGCHLD when its' *brother* 
dies.

echo is a shell built-in so the process triggering the firmware load 
(the shell) and the process dying (sleep) *are* father and child.

Martin