From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: [PATCH 1/10] Add generic helpers for arch IPI function calls Date: Wed, 11 Jun 2008 05:25:31 +0200 Message-ID: <20080611032531.GC8228@wotan.suse.de> References: <1212051504-12561-1-git-send-email-jens.axboe@oracle.com> <1212051504-12561-2-git-send-email-jens.axboe@oracle.com> <1213109485.24701.107.camel@pc1117.cambridge.arm.com> <20080610154716.GF15481@linux.vnet.ibm.com> <1213116788.24701.133.camel@pc1117.cambridge.arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from ns2.suse.de ([195.135.220.15]:48968 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754546AbYFKDZh (ORCPT ); Tue, 10 Jun 2008 23:25:37 -0400 Content-Disposition: inline In-Reply-To: <1213116788.24701.133.camel@pc1117.cambridge.arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Catalin Marinas Cc: paulmck@linux.vnet.ibm.com, Jens Axboe , linux-kernel@vger.kernel.org, peterz@infradead.org, linux-arch@vger.kernel.org, jeremy@goop.org, mingo@elte.hu, Russell King On Tue, Jun 10, 2008 at 05:53:08PM +0100, Catalin Marinas wrote: > On Tue, 2008-06-10 at 08:47 -0700, Paul E. McKenney wrote: > > On Tue, Jun 10, 2008 at 03:51:25PM +0100, Catalin Marinas wrote: > > > I was thinking whether this condition can be removed and allow the > > > smp_call_function*() to be called with IRQs disabled. At a quick look, > > > it seems to be possible if the csd_flag_wait() function calls the IPI > > > handlers directly when the IRQs are disabled (see the patch below). > [...] > > There were objections last month: http://lkml.org/lkml/2008/5/3/167 > > Thanks, I missed this discussion. > > > The issue was that this permits some interrupts to arrive despite > > interrupts being disabled. There seemed to be less resistance to > > doing this in the wait==1 case, however. > > The "(wait == 1) && irqs_disabled()" case is what I would be interested > in. In the patch you proposed, this doesn't seem to be allowed (at least > from the use of WARN_ON). However, from your post in May: > > > 5. If you call smp_call_function() with irqs disabled, then you > > are guaranteed that no other CPU's smp_call_function() handler > > will be invoked while smp_call_function() is executing. > > this would be possible but no one need this functionality yet. > > Would one use-case (ARM SMP and DMA cache maintenance) be enough to > implement this or I should add it to the ARM-specific code? How will you implement it? You have to be able to wait *somewhere* (either before or after the smp_call_function call) with interrupts enabled. It is not enough just to eg. use a spinlock around smp_call_function, because other CPUs might also be trying to call down the same path also with interrupts disabled, and they'll wait forever on the spinlock.