linux-kernel - Re: [PATCH] usbnet: Fix two races between usbnet

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Message-Id: <20150824.104328.554582952440857559.davem@davemloft.net>
Date:	Mon, 24 Aug 2015 10:43:28 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eugene.shatokhin@...alab.ru
Cc:	bjorn@...k.no, oneukum@...e.com, netdev@...r.kernel.org,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usbnet: Fix two races between usbnet_stop() and the BH

From: Eugene Shatokhin <eugene.shatokhin@...alab.ru>
Date: Wed, 19 Aug 2015 14:59:01 +0300

> So the following might be possible, although unlikely:
> 
> CPU0             CPU1
>                  clear_bit: read dev->flags
>                  clear_bit: clear EVENT_RX_KILL in the read value
> 
> dev->flags=0;
> 
>                  clear_bit: write updated dev->flags
> 
> As a result, dev->flags may become non-zero again.

Is this really possible?

Stores really are "atomic" in the sense that the do their update
in one indivisible operation.

Atomic operations like clear_bit also will behave that way.

If a clear_bit is in progress, the "dev->flags=0" store will not be
able to grab the cache line exclusively until the clear_bit is done.

So I think the above sequent of events is completely impossible.  Once
a clear_bit starts, a write by another foreign agent on the bus is
absolutely impossible to legally occur until the clear_bit completes.

I think this is a non-issue.
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