From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks Date: Mon, 18 Feb 2013 23:26:22 +0530 Message-ID: <51226B46.9080707@linux.vnet.ibm.com> References: <20130218123714.26245.61816.stgit@srivatsabhat.in.ibm.com> <20130218123856.26245.46705.stgit@srivatsabhat.in.ibm.com> <5122551E.1080703@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: linux-doc@vger.kernel.org, peterz@infradead.org, fweisbec@gmail.com, linux-kernel@vger.kernel.org, mingo@kernel.org, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, rusty@rustcorp.com.au, rostedt@goodmis.org, rjw@sisk.pl, namhyung@kernel.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, oleg@redhat.com, vincent.guittot@linaro.org, sbw@mit.edu, tj@kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org To: Michel Lespinasse Return-path: In-Reply-To: <5122551E.1080703@linux.vnet.ibm.com> Sender: linux-doc-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 02/18/2013 09:51 PM, Srivatsa S. Bhat wrote: > Hi Michel, > > On 02/18/2013 09:15 PM, Michel Lespinasse wrote: >> Hi Srivasta, >> >> I admit not having followed in detail the threads about the previous >> iteration, so some of my comments may have been discussed already >> before - apologies if that is the case. >> >> On Mon, Feb 18, 2013 at 8:38 PM, Srivatsa S. Bhat >> wrote: >>> Reader-writer locks and per-cpu counters are recursive, so they can be >>> used in a nested fashion in the reader-path, which makes per-CPU rwlocks also >>> recursive. Also, this design of switching the synchronization scheme ensures >>> that you can safely nest and use these locks in a very flexible manner. [...] >>> void percpu_write_lock(struct percpu_rwlock *pcpu_rwlock) >>> { >>> + unsigned int cpu; >>> + >>> + /* >>> + * Tell all readers that a writer is becoming active, so that they >>> + * start switching over to the global rwlock. >>> + */ >>> + for_each_possible_cpu(cpu) >>> + per_cpu_ptr(pcpu_rwlock->rw_state, cpu)->writer_signal = true; >> >> I don't see anything preventing a race with the corresponding code in >> percpu_write_unlock() that sets writer_signal back to false. Did I >> miss something here ? It seems to me we don't have any guarantee that >> all writer signals will be set to true at the end of the loop... >> > > Ah, thanks for pointing that out! IIRC Oleg had pointed this issue in the last > version, but back then, I hadn't fully understood what he meant. Your > explanation made it clear. I'll work on fixing this. > We can fix this by using the simple patch (untested) shown below. The alternative would be to acquire the rwlock for write, update the ->writer_signal values, release the lock, wait for readers to switch, again acquire the rwlock for write with interrupts disabled etc... which makes it kinda messy, IMHO. So I prefer the simple version shown below. diff --git a/lib/percpu-rwlock.c b/lib/percpu-rwlock.c index bf95e40..64ccd3f 100644 --- a/lib/percpu-rwlock.c +++ b/lib/percpu-rwlock.c @@ -50,6 +50,12 @@ (__this_cpu_read((pcpu_rwlock)->rw_state->writer_signal)) +/* + * Spinlock to synchronize access to the writer's data-structures + * (->writer_signal) from multiple writers. + */ +static DEFINE_SPINLOCK(writer_side_lock); + int __percpu_init_rwlock(struct percpu_rwlock *pcpu_rwlock, const char *name, struct lock_class_key *rwlock_key) { @@ -191,6 +197,8 @@ void percpu_write_lock_irqsave(struct percpu_rwlock *pcpu_rwlock, { unsigned int cpu; + spin_lock(&writer_side_lock); + /* * Tell all readers that a writer is becoming active, so that they * start switching over to the global rwlock. @@ -252,5 +260,6 @@ void percpu_write_unlock_irqrestore(struct percpu_rwlock *pcpu_rwlock, per_cpu_ptr(pcpu_rwlock->rw_state, cpu)->writer_signal = false; write_unlock_irqrestore(&pcpu_rwlock->global_rwlock, *flags); + spin_unlock(&writer_side_lock); }