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

[Martin Fuzzey <mfuzzey@parkeon.com>]

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


Martin