From mboxrd@z Thu Jan 1 00:00:00 1970 From: George Dunlap Subject: Re: [PATCH] x86: fix ordering of operations in destroy_irq() Date: Thu, 30 May 2013 17:51:47 +0100 Message-ID: <51A783A3.6080703@eu.citrix.com> References: <51A5C33A02000078000D974A@nat28.tlf.novell.com> <51A77CEB.6030409@eu.citrix.com> <51A78F9D020000780009B71B@nat28.tlf.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <51A78F9D020000780009B71B@nat28.tlf.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich Cc: andrew.cooper3@citrix.com, keir@xen.org, xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org On 05/30/2013 05:42 PM, Jan Beulich wrote: >>>> George Dunlap 05/30/13 6:23 PM >>> >> On 05/29/2013 07:58 AM, Jan Beulich wrote: >>> The fix for XSA-36, switching the default of vector map management to >>> be per-device, exposed more readily a problem with the cleanup of these >>> vector maps: dynamic_irq_cleanup() clearing desc->arch.used_vectors >>> keeps the subsequently invoked clear_irq_vector() from clearing the >>> bits for both the in-use and a possibly still outstanding old vector. >>> >>> Fix this by folding dynamic_irq_cleanup() into destroy_irq(), which was >>> its only caller, deferring the clearing of the vector map pointer until >>> after clear_irq_vector(). >>> >>> Once at it, also defer resetting of desc->handler until after the loop >>> around smp_mb() checking for IRQ_INPROGRESS to be clear, fixing a >>> (mostly theoretical) issue with the intercation with do_IRQ(): If we >>> don't defer the pointer reset, do_IRQ() could, for non-guest IRQs, call >>> ->ack() and ->end() with different ->handler pointers, potentially >>> leading to an IRQ remaining un-acked. The issue is mostly theoretical >>> because non-guest IRQs are subject to destroy_irq() only on (boot time) >>> error paths. >>> >>> As to the changed locking: Invoking clear_irq_vector() with desc->lock >>> held is okay because vector_lock already nests inside desc->lock (proven >>> by set_desc_affinity(), which takes vector_lock and gets called from >>> various desc->handler->ack implementations, getting invoked with >>> desc->lock held). >>> >>> Reported-by: Andrew Cooper >>> Signed-off-by: Jan Beulich >> >> How big of an impact is this bug? How many people are actually affected >> by it? > > Andrew will likely be able to give you more precise info on this, but this > fixes a problem observed in practice. Any AMD system with IOMMU would > be affected. > >> It's a bit hard for me to tell from the description, but it looks like >> it's code motion, then some "theoretical" issues. > > No, the description is pretty precise here: It fixes an actual issue and, > along the way, also a theoretical one. > >> Is the improvement this patch represents worth the potential risk of >> bugs at this point? > > I think so - otherwise it would need to be backported right away after the > release. Right -- then if you could also commit this tomorrow, it will get the best testing we can give it. :-) Acked-by: George Dunlap -George