From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Date: Sat, 08 Dec 2012 00:03:16 +0530 Message-ID: <50C2366C.3000204@linux.vnet.ibm.com> References: <20121207173702.27305.1486.stgit@srivatsabhat.in.ibm.com> <20121207173759.27305.84316.stgit@srivatsabhat.in.ibm.com> <20121207175724.GA2821@htj.dyndns.org> <20121207181608.GB2821@htj.dyndns.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from e23smtp07.au.ibm.com ([202.81.31.140]:39933 "EHLO e23smtp07.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1423749Ab2LGSeu (ORCPT ); Fri, 7 Dec 2012 13:34:50 -0500 Received: from /spool/local by e23smtp07.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Sat, 8 Dec 2012 04:30:00 +1000 In-Reply-To: <20121207181608.GB2821@htj.dyndns.org> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Tejun Heo Cc: tglx@linutronix.de, peterz@infradead.org, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, akpm@linux-foundation.org, namhyung@kernel.org, vincent.guittot@linaro.org, oleg@redhat.com, sbw@mit.edu, amit.kucheria@linaro.org, rostedt@goodmis.org, rjw@sisk.pl, wangyun@linux.vnet.ibm.com, xiaoguangrong@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org On 12/07/2012 11:46 PM, Tejun Heo wrote: > Hello, again. > > On Fri, Dec 07, 2012 at 09:57:24AM -0800, Tejun Heo wrote: >> possible. Also, I think the right approach would be auditing each >> get_online_cpus_atomic() callsites and figure out proper locking order >> rather than implementing a construct this unusual especially as >> hunting down the incorrect cases shouldn't be difficult given proper >> lockdep annotation. > > On the second look, it looks like you're implementing proper > percpu_rwlock semantics Ah, nice! I didn't realize that I was actually doing what I intended to avoid! ;-) Looking at the implementation of lglocks, and going by Oleg's earlier comment that we just need to replace spinlock_t with rwlock_t in them to get percpu_rwlocks, I was horrified at the kinds of circular locking dependencies that they would be prone to, unless used carefully. So I devised this scheme to be safe, while still having relaxed rules. But if *this* is what percpu_rwlocks should ideally look like, then great! :-) > as readers aren't supposed to induce circular > dependency directly. Yep, in this scheme, nobody will end up in circular dependency. > Can you please work with Oleg to implement > proper percpu-rwlock and use that for CPU hotplug rather than > implementing it inside CPU hotplug? > Sure, I'd be more than happy to! Regards, Srivatsa S. Bhat