From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steven Rostedt Subject: Re: [PATCH v5 01/45] percpu_rwlock: Introduce the global reader-writer lock backend Date: Tue, 22 Jan 2013 14:32:32 -0500 Message-ID: <1358883152.21576.55.camel@gandalf.local.home> References: <20130122073210.13822.50434.stgit@srivatsabhat.in.ibm.com> <20130122073315.13822.27093.stgit@srivatsabhat.in.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-15" Content-Transfer-Encoding: 7bit Cc: tglx@linutronix.de, peterz@infradead.org, tj@kernel.org, oleg@redhat.com, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, akpm@linux-foundation.org, namhyung@kernel.org, wangyun@linux.vnet.ibm.com, xiaoguangrong@linux.vnet.ibm.com, rjw@sisk.pl, sbw@mit.edu, fweisbec@gmail.com, linux@arm.linux.org.uk, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org To: "Srivatsa S. Bhat" Return-path: In-Reply-To: <20130122073315.13822.27093.stgit@srivatsabhat.in.ibm.com> Sender: linux-arch-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, 2013-01-22 at 13:03 +0530, Srivatsa S. Bhat wrote: > A straight-forward (and obvious) algorithm to implement Per-CPU Reader-Writer > locks can also lead to too many deadlock possibilities which can make it very > hard/impossible to use. This is explained in the example below, which helps > justify the need for a different algorithm to implement flexible Per-CPU > Reader-Writer locks. > > We can use global rwlocks as shown below safely, without fear of deadlocks: > > Readers: > > CPU 0 CPU 1 > ------ ------ > > 1. spin_lock(&random_lock); read_lock(&my_rwlock); > > > 2. read_lock(&my_rwlock); spin_lock(&random_lock); > > > Writer: > > CPU 2: > ------ > > write_lock(&my_rwlock); > I thought global locks are now fair. That is, a reader will block if a writer is waiting. Hence, the above should deadlock on the current rwlock_t types. We need to fix those locations (or better yet, remove all rwlocks ;-) -- Steve