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

On Mon, 24 Aug 2015, Alan Stern wrote:

On Mon, 24 Aug 2015, David Miller wrote:

quoted

From: Eugene Shatokhin <redacted>
Date: Wed, 19 Aug 2015 14:59:01 +0300

quoted

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.

quoted

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.

Now that I see this in writing, I realize it's not possible after all.  
clear_bit() et al. will work with a single unsigned long, which doesn't
leave any place for spinlocks or other mechanisms.  I was thinking of 
atomic_t.

So never mind...

Alan Stern