Re: [RFC PATCH 2/3] usbnet: Avoid potential races in usbnet_deferred_kevent()

On Tue, Sep 19, 2017 at 1:37 PM, Oliver Neukum [off-list ref] wrote:

Am Dienstag, den 19.09.2017, 09:15 -0700 schrieb Douglas Anderson:

quoted

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.

NOTES:
- No known bugs are fixed by this; it's just found by code inspection.

Hi,

unfortunately the patch is wrong. The flags must be cleared only
in case the handler is successful. That is not guaranteed.

Just out of curiosity, what is the retry mechanism ? Whenever a new,
possibly unrelated, event is scheduled ?

Thanks,
Guenter