linux-kernel - Re: [patch] x86_64, irq: check remote IRR bit before migrating level triggered irq

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Message-ID: <20070518183941.GA28957@linux-os.sc.intel.com>
Date:	Fri, 18 May 2007 11:39:41 -0700
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	Yinghai Lu <yhlu.kernel@...il.com>
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>, mingo@...e.hu,
	ak@...e.de, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, "Zou, Nanhai" <nanhai.zou@...el.com>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>,
	"Packard, Keith" <keith.packard@...el.com>
Subject: Re: [patch] x86_64, irq: check remote IRR bit before migrating level triggered irq

On Fri, May 18, 2007 at 11:28:25AM -0700, Yinghai Lu wrote:
> On 5/18/07, Siddha, Suresh B <suresh.b.siddha@...el.com> wrote:
> >
> > If the vector number stays same during irq migration and if we reset remote
> > IRR bit using the above method(edge and then back to level) during
> > irq migration, then we have a problem. A new interrupt arriving on a new
> > cpu will set the remote IRR bit and now the old inflight EOI broadcast
> > reaches IOAPIC RTE(resetting the remote IRR bit, because the vector in the
> > broadcast msg is same), while the kernel code still assumes that the remote
> > IRR bit is still set. This will lead to more problems and issues.
> 
> coud add some line __assign_irq_vector. to make sure old_vector!=vector.

hmm..
what happens when there is second(and very quick) irq migration which brings the
irq back to old cpu(or to a third cpu) with old vector.

Point is, we are not taking care of the inflight messages(which can perhaps,
theoretically, can get delayed for long time)

thanks,
suresh
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