linux-kernel - Re: [RFC PATCH 2/3] usbnet: Avoid potential races in usbnet_deferred

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Date:   Tue, 19 Sep 2017 22:37:56 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     groeck@...omium.org, grundler@...omium.org,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [RFC PATCH 2/3] usbnet: Avoid potential races in
 usbnet_deferred_kevent()

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.
> 
> 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.

	Regards
		Oliver

NACK