From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752173Ab3BRSQz (ORCPT ); Mon, 18 Feb 2013 13:16:55 -0500 Received: from e23smtp09.au.ibm.com ([202.81.31.142]:50117 "EHLO e23smtp09.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751402Ab3BRSQv (ORCPT ); Mon, 18 Feb 2013 13:16:51 -0500 Message-ID: <51226F91.7000108@linux.vnet.ibm.com> Date: Mon, 18 Feb 2013 23:44:41 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 MIME-Version: 1.0 To: Michel Lespinasse CC: linux-doc@vger.kernel.org, peterz@infradead.org, fweisbec@gmail.com, linux-kernel@vger.kernel.org, namhyung@kernel.org, mingo@kernel.org, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, rusty@rustcorp.com.au, rostedt@goodmis.org, rjw@sisk.pl, vincent.guittot@linaro.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, oleg@redhat.com, sbw@mit.edu, tj@kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks References: <20130218123714.26245.61816.stgit@srivatsabhat.in.ibm.com> <20130218123856.26245.46705.stgit@srivatsabhat.in.ibm.com> <5122551E.1080703@linux.vnet.ibm.com> <51226B46.9080707@linux.vnet.ibm.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13021818-3568-0000-0000-0000032C798F Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/18/2013 11:37 PM, Michel Lespinasse wrote: > On Tue, Feb 19, 2013 at 1:56 AM, Srivatsa S. Bhat > wrote: >> On 02/18/2013 09:51 PM, Srivatsa S. Bhat wrote: >>> On 02/18/2013 09:15 PM, Michel Lespinasse wrote: >>>> I don't see anything preventing a race with the corresponding code in >>>> percpu_write_unlock() that sets writer_signal back to false. Did I >>>> miss something here ? It seems to me we don't have any guarantee that >>>> all writer signals will be set to true at the end of the loop... >>> >>> Ah, thanks for pointing that out! IIRC Oleg had pointed this issue in the last >>> version, but back then, I hadn't fully understood what he meant. Your >>> explanation made it clear. I'll work on fixing this. >> >> We can fix this by using the simple patch (untested) shown below. >> The alternative would be to acquire the rwlock for write, update the >> ->writer_signal values, release the lock, wait for readers to switch, >> again acquire the rwlock for write with interrupts disabled etc... which >> makes it kinda messy, IMHO. So I prefer the simple version shown below. > > Looks good. > > Another alternative would be to make writer_signal an atomic integer > instead of a bool. That way writers can increment it before locking > and decrement it while unlocking. > Yep, that would also do. But the spinlock version looks simpler - no need to check if the atomic counter is non-zero, no need to explicitly spin in a tight-loop etc. > To reduce the number of atomic ops during writer lock/unlock, the > writer_signal could also be a global read_mostly variable (I don't see > any downsides to that compared to having it percpu - or is it because > you wanted all the fastpath state to be in one single cacheline ?) > Yes, we (Oleg and I) debated for a while about global vs percpu, and then finally decided to go with percpu to have cache benefits. Regards, Srivatsa S. Bhat