Re: [PATCH] usbnet: Fix two races between usbnet_stop() and the BH

From: David Miller
Date: Mon Aug 24 2015 - 13:43:48 EST

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

> So the following might be possible, although unlikely:
> 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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