Re: [RFC PATCH 2/3] usbnet: Avoid potential races in usbnet_deferred_kevent()
From: Oliver Neukum
Date: Tue Sep 19 2017 - 16:41:29 EST
Am Dienstag, den 19.09.2017, 09:15 -0700 schrieb Douglas Anderson:
> In general when you've got a flag communicating that "something needs
> to be done" you want to clear that flag _before_ doing the task. If
> you clear the flag _after_ doing the task you end up with the risk
> that this will happen:
> 1. Requester sets flag saying task A needs to be done.
> 2. Worker comes and stars doing task A.
> 3. Worker finishes task A but hasn't yet cleared the flag.
> 4. Requester wants to set flag saying task A needs to be done again.
> 5. Worker clears the flag without doing anything.
> Let's make the usbnet codebase consistently clear the flag _before_ it
> does the requested work. That way if there's another request to do
> the work while the work is already in progress it won't be lost.
> - No known bugs are fixed by this; it's just found by code inspection.
unfortunately the patch is wrong. The flags must be cleared only
in case the handler is successful. That is not guaranteed.