From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keir Fraser Subject: Re: [xen-unstable test] 6947: regressions - trouble: broken/fail/pass Date: Tue, 03 May 2011 11:09:33 +0100 Message-ID: References: <4DBFE886020000780003F5AD@vpn.id2.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4DBFE886020000780003F5AD@vpn.id2.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Jan Beulich Cc: xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org On 03/05/2011 10:35, "Jan Beulich" wrote: >> Oh, another way would be to make lookup_slot invocations from IRQ context be >> RCU-safe. Then the radix tree updates would not have to synchronise on the >> irq_desc lock? And I believe Linux has examples of RCU-safe usage of radix >> trees -- certainly Linux's radix-tree.h mentions RCU. >> >> I must say this would be far more attractive to me than hacking the xmalloc >> subsystem. That's pretty nasty. > > I think that I can actually get away with two stage insertion/removal > without needing RCU, based on the fact that prior to these changes > we have the translation arrays also hold zero values that mean "does > not have a valid translation". Hence I can do tree insertion (removal) > with just d->event_lock held, but data not yet (no longer) populated, > and valid <-> invalid transitions only happening with the IRQ's > descriptor lock held (and interrupts disabled). All this requires is that > readers properly deal with the non-populated state, which they > already had to in the first version of the patch anyway. But the readers in irq context will call lookup_slot() without d->event_lock held? In that case you do need an RCU-aware version of radix-tree.[ch], because lookups can be occurring concurrently with insertions/deletions. Good news is that the RCU-aware radix tree implementation hides the RCU details from you entirely. Well, in any case, I'm happy to iterate on this patch if necessary. -- Keir