Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753314Ab2K2RZt (ORCPT ); Thu, 29 Nov 2012 12:25:49 -0500 Received: from mx1.redhat.com ([209.132.183.28]:22459 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752464Ab2K2RZr (ORCPT ); Thu, 29 Nov 2012 12:25:47 -0500 Date: Thu, 29 Nov 2012 19:25:41 +0200 From: Gleb Natapov To: Frederic Weisbecker Cc: Li Zhong , linux-next list , LKML , paulmck@linux.vnet.ibm.com, sasha.levin@oracle.com, avi@redhat.com Subject: Re: [RFC PATCH v2] Add rcu user eqs exception hooks for async page fault Message-ID: <20121129172541.GA20873@redhat.com> References: <1353993325.14050.49.camel@ThinkPad-T5421.cn.ibm.com> <1354090704.3054.13.camel@ThinkPad-T5421.cn.ibm.com> <1354153788.2499.9.camel@ThinkPad-T5421.cn.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2549 Lines: 55 On Thu, Nov 29, 2012 at 03:40:05PM +0100, Frederic Weisbecker wrote: > 2012/11/29 Li Zhong : > > On Wed, 2012-11-28 at 13:55 +0100, Frederic Weisbecker wrote: > >> > With rcu_user_exit() at the beginning, now rcu_irq_enter() only protects > >> > the cpu idle eqs, but it's not good to call rcu_irq_exit() after the cpu > >> > halt and the page ready. > >> > >> Hmm, why is it not good? > > > > I think in this case, as cpu halts and waits for page ready, it is > > really in idle, and it's better for rcu to think it as rcu idle. But > > with rcu_irq_enter(), the rcu would not think this cpu as idle. And the > > rcu_irq_exit() is only called after this idle period to mark this cpu as > > rcu idle again. > > > Indeed. How about calling rcu_irq_exit() before native_safe_halt() and > rcu_irq_enter() right after? > We can't. If exception happens in the middle of rcu read section while preemption is disabled then calling rcu_irq_exit() before halt is incorrect. We can do that only if exception happen during idle and this should be rare enough for us to not care. > >> > So I still want to remove it. And later if it shows that we really needs > >> > rcu somewhere in this code path, maybe we could use RCU_NONIDLE() to > >> > protect it. ( The suspicious RCU usage reported in commit > >> > c5e015d4949aa665 seems related to schedule(), which is not in the code > >> > path if we are in cpu idle eqs ) > >> > >> Yes but if rcu_irq_*() calls are fine to be called there, and I > >> believe they are because exception_enter() exits the user mode, we > >> should start to protect there right now instead of waiting for a > >> potential future warning of illegal RCU use. > > > > I agree, but I think by only protecting the necessary code avoids > > marking the entire waiting period as rcu non idle. > > If we use RCU_NONIDLE(), this assume we need to check all the code > there deeply for potential RCU uses and ensure it will never be > extended later to use RCU. We really don't scale enough for that. > There are too much subsystems involved there: waitqueue, spinlocks, > slab, per cpu, etc... > > So I strongly suggest we use rcu_irq_*() APIs, and relax them around > native_safe_halt() like I suggested above. This seems to me the safest > solution. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/