Re: [PATCH]generic usb serial driver

On Friday 30 March 2007 2:49 pm, Oliver Neukum wrote:

> > Nope.  If an smp_mb() is needed, the reason is because of other
> > driver-internal bugs.

> > Remember that the driver isn't allowed to so much as *LOOK* at any
> > field of an URB until the completion callback.  That generalizes:
> > it mustn't do so until that callback (cleanly) hands the URB back
> 
> Yes and the callback is guaranteed that all the urb's fields are
> valid. Which is the whole point. Is that guarantee only valid to the
> callback or to the whole driver?
> 
> CPU A (callback)			CPU B
> 
> flag = 1;					if (!flag) barrier();
> a = urb->status;			b = urb->status;
> 
> Do we guarantee a==b or don't we?

I don't see a handoff there that's even vaguely correct, which
means this code displays one of those "driver-internal bugs".

So of course there's no such guarantee.

It's perfectly legit for CPU A to reorder writes; common, even,
when they're in different cache lines.


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