Re: [PATCH 00/10] usb-serial: Switches from spin lock to atomic_t.

[Eduardo Pereira Habkost]

Hi, Greg,

On Wed, Dec 07, 2005 at 09:56:14AM -0800, Greg KH wrote:
> On Wed, Dec 07, 2005 at 03:13:32PM -0200, Eduardo Pereira Habkost wrote:
> > I have a small question: in my view, this patch series is a small
> > step towards implementing the usb-serial drivers The Right Way, as it
> > removes a a bit of duplicated code.
> 
> It doesn't remove any "duplicated code", it only changes a spinlock to
> an atomic_t for one single value (which I personally do not think is the
> best thing to do, and based on the number of comments on this thread, I
> think others also feel this way.)

Every usb-serial driver currently has:

spin_lock(port->lock);
if (port->write_urb_busy)
	return;
port->write_urb_busy = 1;
spin_unlock(port->lock);


Having the same 5 lines on many usb-serial drivers looks like duplicated
code to me. Maybe I am being too exigent. Anyway, this is unrelated to
the atomic_t change, and we could do it without dropping ths spinlock.

But I would like to know: would you would accept such change (that
encapsulate this write_urb_busy logic without necessarily dropping the
spinlock)?


And, about the atomic_t: I've felt most people didn't liked the atomic_t
approach for one of two reasons:

1. The existence of write_urb_busy looks like a hack, and we've
   make it explicit when we isolated the code that uses write_urb_busy
   in a set of functions (the point of Arjan in his "square wheel"
   comment)
2. The whole discussion about atomic_t vs. spinlock efficiency


I agree with (1), but I still don't see why using a spinlock to protect
a single field is better than using atomic_t. Both in code readability
and efficiency. Specially as the difference between each one (even which
one is more efficient) is very arch-specific.


Thank you very much for your advice,

-- 
Eduardo

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