linux-kernel - Re: Memory synchronization vs. interrupt handlers

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Date:	Thu, 29 Aug 2013 16:51:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Russell King <linux@....linux.org.uk>,
	Ingo Molnar <mingo@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Ming Lei <ming.lei@...onical.com>,
	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	arnd.bergmann@...aro.org, olof@...om.net, benh@...nel.crashing.org
Subject: Re: Memory synchronization vs. interrupt handlers

On Wed, Aug 28, 2013 at 01:28:08PM -0700, H. Peter Anvin wrote:
> On 08/28/2013 12:16 PM, Alan Stern wrote:
> > Russell, Peter, and Ingo:
> > 
> > Can you folks enlighten us regarding this issue for some common 
> > architectures?
> 
> On x86, IRET is a serializing instruction; it guarantees hard
> serialization of absolutely everything.

So a second interrupt from this same device could not appear to happen
before the IRET, no matter what device and/or I/O bus?  Or is IRET
defined to synchronize all the way out to the whatever device is
generating the next interrupt?

> I would expect architectures that have weak memory ordering to put
> appropriate barriers in the IRQ entry/exit code.

Adding a few on CC.  Also restating the question as I understand it:

	Suppose that a given device generates an interrupt on CPU 0,
	but that before CPU 0's interrupt handler completes, this device
	wants to generate a second interrupt on CPU 1.  This can happen
	as soon as CPU 0's handler does an EOI or equivalent.

	Can CPU 1's interrupt handler assume that all the in-memory effects
	of CPU 0's interrupt handler will be visible, even if neither
	interrupt handler uses locking or memory barriers?

							Thanx, Paul

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