From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [PATCH v5 04/45] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks Date: Thu, 24 Jan 2013 10:00:04 +0530 Message-ID: <5100B8CC.4080406@linux.vnet.ibm.com> References: <20130122073210.13822.50434.stgit@srivatsabhat.in.ibm.com> <20130122073347.13822.85876.stgit@srivatsabhat.in.ibm.com> <20130123185522.GG2373@mtj.dyndns.org> <51003B20.2060506@linux.vnet.ibm.com> <20130123195740.GI2373@mtj.dyndns.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130123195740.GI2373@mtj.dyndns.org> Sender: linux-doc-owner@vger.kernel.org To: Tejun Heo Cc: tglx@linutronix.de, peterz@infradead.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, walken@google.com List-Id: linux-pm@vger.kernel.org On 01/24/2013 01:27 AM, Tejun Heo wrote: > Hello, Srivatsa. > > On Thu, Jan 24, 2013 at 01:03:52AM +0530, Srivatsa S. Bhat wrote: >> Hmm.. I split it up into steps to help explain the reasoning behind >> the code sufficiently, rather than spring all of the intricacies at >> one go (which would make it very hard to write the changelog/comments >> also). The split made it easier for me to document it well in the >> changelog, because I could deal with reasonable chunks of code/complexity >> at a time. IMHO that helps people reading it for the first time to >> understand the logic easily. > > I don't know. It's a judgement call I guess. I personally would much > prefer having ample documentation as comments in the source itself or > as a separate Documentation/ file as that's what most people are gonna > be looking at to figure out what's going on. Maybe just compact it a > bit and add more in-line documentation instead? > OK, I'll think about this. >>> The only two options are either punishing writers or identifying and >>> updating all such possible deadlocks. percpu_rwsem does the former, >>> right? I don't know how feasible the latter would be. >> >> I don't think we can avoid looking into all the possible deadlocks, >> as long as we use rwlocks inside get/put_online_cpus_atomic() (assuming >> rwlocks are fair). Even with Oleg's idea of using synchronize_sched() >> at the writer, we still need to take care of locking rules, because the >> synchronize_sched() only helps avoid the memory barriers at the reader, >> and doesn't help get rid of the rwlocks themselves. > > Well, percpu_rwlock don't have to use rwlock for the slow path. It > can implement its own writer starving locking scheme. It's not like > implementing slow path global rwlock logic is difficult. > Great idea! So probably I could use atomic ops or something similar in the slow path to implement the scheme we need... >> CPU 0 CPU 1 >> >> read_lock(&rwlock) >> >> write_lock(&rwlock) //spins, because CPU 0 >> //has acquired the lock for read >> >> read_lock(&rwlock) >> ^^^^^ >> What happens here? Does CPU 0 start spinning (and hence deadlock) or will >> it continue realizing that it already holds the rwlock for read? > > I don't think rwlock allows nesting write lock inside read lock. > read_lock(); write_lock() will always deadlock. > Sure, I understand that :-) My question was, what happens when *two* CPUs are involved, as in, the read_lock() is invoked only on CPU 0 whereas the write_lock() is invoked on CPU 1. For example, the same scenario shown above, but with slightly different timing, will NOT result in a deadlock: Scenario 2: CPU 0 CPU 1 read_lock(&rwlock) read_lock(&rwlock) //doesn't spin write_lock(&rwlock) //spins, because CPU 0 //has acquired the lock for read So I was wondering whether the "fairness" logic of rwlocks would cause the second read_lock() to spin (in the first scenario shown above) because a writer is already waiting (and hence new readers should spin) and thus cause a deadlock. Regards, Srivatsa S. Bhat