From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Howells Subject: Re: [PATCH v5 01/45] percpu_rwlock: Introduce the global reader-writer lock backend Date: Mon, 11 Feb 2013 12:41:31 +0000 Message-ID: <30708.1360586491@warthog.procyon.org.uk> References: <20130122073315.13822.27093.stgit@srivatsabhat.in.ibm.com> <20130122073210.13822.50434.stgit@srivatsabhat.in.ibm.com> Return-path: In-Reply-To: <20130122073315.13822.27093.stgit@srivatsabhat.in.ibm.com> Sender: linux-kernel-owner@vger.kernel.org To: "Srivatsa S. Bhat" Cc: dhowells@redhat.com, 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, rostedt@goodmis.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 List-Id: linux-arch.vger.kernel.org Srivatsa S. Bhat wrote: > 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); The lock order on CPU 0 is unsafe if CPU2 can do: write_lock(&my_rwlock); spin_lock(&random_lock); and on CPU 1 if CPU2 can do: spin_lock(&random_lock); write_lock(&my_rwlock); I presume you were specifically excluding these situations? David From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:33973 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756914Ab3BKMmO (ORCPT ); Mon, 11 Feb 2013 07:42:14 -0500 From: David Howells In-Reply-To: <20130122073315.13822.27093.stgit@srivatsabhat.in.ibm.com> References: <20130122073315.13822.27093.stgit@srivatsabhat.in.ibm.com> <20130122073210.13822.50434.stgit@srivatsabhat.in.ibm.com> Subject: Re: [PATCH v5 01/45] percpu_rwlock: Introduce the global reader-writer lock backend Date: Mon, 11 Feb 2013 12:41:31 +0000 Message-ID: <30708.1360586491@warthog.procyon.org.uk> Sender: linux-arch-owner@vger.kernel.org List-ID: To: "Srivatsa S. Bhat" Cc: dhowells@redhat.com, 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, rostedt@goodmis.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 Message-ID: <20130211124131.IhXDCOPUJH1wxDKJ4BMdjn4zcxuNpEqa4lOHJ6NrWic@z> Srivatsa S. Bhat wrote: > 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); The lock order on CPU 0 is unsafe if CPU2 can do: write_lock(&my_rwlock); spin_lock(&random_lock); and on CPU 1 if CPU2 can do: spin_lock(&random_lock); write_lock(&my_rwlock); I presume you were specifically excluding these situations? David