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