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Date: Thu, 24 Jan 2013 10:00:04 +0530 From: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> To: Tejun Heo <tj@...nel.org> CC: tglx@...utronix.de, peterz@...radead.org, oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au, mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org, rostedt@...dmis.org, wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com, rjw@...k.pl, sbw@....edu, fweisbec@...il.com, linux@....linux.org.uk, nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org, linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, walken@...gle.com Subject: Re: [PATCH v5 04/45] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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