Re: [PATCH 0/4] firmware: fix fallback mechanism by ignoring SIGCHLD

[Martin Fuzzey]

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