linux-kernel - Re: [linux-usb-devel] question on irqs and memory ordering

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Message-ID: <Pine.LNX.4.44L0.0703141557210.7723-100000@iolanthe.rowland.org>
Date:	Wed, 14 Mar 2007 16:06:28 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	David Howells <dhowells@...hat.com>,
	USB development list <linux-usb-devel@...ts.sourceforge.net>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [linux-usb-devel] question on irqs and memory ordering

On Wed, 14 Mar 2007, Oliver Neukum wrote:

> Hi,
> 
> I am looking at this code:
> 
> 	if (new_msr &
> 	    (MOS_MSR_DELTA_CTS | MOS_MSR_DELTA_DSR | MOS_MSR_DELTA_RI |
> 	     MOS_MSR_DELTA_CD)) {
> 		icount = &mos7840_port->icount;
> 
> 		/* update input line counters */
> 		if (new_msr & MOS_MSR_DELTA_CTS) {
> 			icount->cts++;
> 		}
> 		if (new_msr & MOS_MSR_DELTA_DSR) {
> 			icount->dsr++;
> 		}
> 		if (new_msr & MOS_MSR_DELTA_CD) {
> 			icount->dcd++;
> 		}
> 		if (new_msr & MOS_MSR_DELTA_RI) {
> 			icount->rng++;
> 		}
> 	}
> 
> It is called in interrupt and uses no locking. What happens if the next
> irq is processed on another cpu? Is that cpu guaranteed to see the updates
> to the incremented variables?

I _think_ it's okay.  Taking an interrupt and returning from an interrupt
ought to have explicit or implicit memory barriers.

It wouldn't hurt to ask someone else.  David Howells wrote
Documentation/memory-barriers.txt so maybe he knows.  (The document itself
doesn't say one way or the other.)

Alan Stern

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