From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [PATCH v5 06/45] percpu_rwlock: Allow writers to be readers, and add lockdep annotations Date: Mon, 11 Feb 2013 01:02:05 +0530 Message-ID: <5117F5B5.6090701@linux.vnet.ibm.com> References: <20130122073210.13822.50434.stgit@srivatsabhat.in.ibm.com> <20130122073416.13822.96504.stgit@srivatsabhat.in.ibm.com> <20130208234754.GM2666@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: tglx@linutronix.de, peterz@infradead.org, tj@kernel.org, oleg@redhat.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 To: paulmck@linux.vnet.ibm.com Return-path: In-Reply-To: <20130208234754.GM2666@linux.vnet.ibm.com> Sender: linux-pm-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 02/09/2013 05:17 AM, Paul E. McKenney wrote: > On Tue, Jan 22, 2013 at 01:04:23PM +0530, Srivatsa S. Bhat wrote: >> CPU hotplug (which will be the first user of per-CPU rwlocks) has a special >> requirement with respect to locking: the writer, after acquiring the per-CPU >> rwlock for write, must be allowed to take the same lock for read, without >> deadlocking and without getting complaints from lockdep. In comparison, this >> is similar to what get_online_cpus()/put_online_cpus() does today: it allows >> a hotplug writer (who holds the cpu_hotplug.lock mutex) to invoke it without >> locking issues, because it silently returns if the caller is the hotplug >> writer itself. >> >> This can be easily achieved with per-CPU rwlocks as well (even without a >> "is this a writer?" check) by incrementing the per-CPU refcount of the writer >> immediately after taking the global rwlock for write, and then decrementing >> the per-CPU refcount before releasing the global rwlock. >> This ensures that any reader that comes along on that CPU while the writer is >> active (on that same CPU), notices the non-zero value of the nested counter >> and assumes that it is a nested read-side critical section and proceeds by >> just incrementing the refcount. Thus we prevent the reader from taking the >> global rwlock for read, which prevents the writer from deadlocking itself. >> >> Add that support and teach lockdep about this special locking scheme so >> that it knows that this sort of usage is valid. Also add the required lockdep >> annotations to enable it to detect common locking problems with per-CPU >> rwlocks. > > Very nice! The write-side interrupt disabling ensures that the task > stays on CPU, as required. > > One request: Could we please have a comment explaining the reasons for > the writer incrementing and decrementing the reader reference count? > > It looked really really strange to me until I came back and read the > commit log. ;-) > Sure :-) Regards, Srivatsa S. Bhat