Date: Mon, 24 Aug 2015 14:06:15 -0400 (EDT)
From: Alan Stern <stern@rowland.harvard.edu>
To: David Miller <davem@davemloft.net>
cc: eugene.shatokhin@rosalab.ru, <bjorn@mork.no>, <oneukum@suse.com>,
        <netdev@vger.kernel.org>, <linux-usb@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] usbnet: Fix two races between usbnet_stop() and the BH
In-Reply-To: <20150824.104328.554582952440857559.davem@davemloft.net>
Message-ID: <Pine.LNX.4.44L0.1508241403100.5935-100000@iolanthe.rowland.org>
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On Mon, 24 Aug 2015, David Miller wrote:

> From: Eugene Shatokhin <eugene.shatokhin@rosalab.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.

Provided you use ACCESS_ONCE or WRITE_ONCE or whatever people like to 
call it now.

> Atomic operations like clear_bit also will behave that way.

Are you certain about that?  I couldn't find any mention of it in
Documentation/atomic_ops.txt.

In theory, an architecture could implement atomic bit operations using 
a spinlock to insure atomicity.  I don't know if any architectures do 
this, but if they do then the scenario above could arise.

Alan Stern

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