Messages in this thread | | | Date | Fri, 9 Nov 2012 19:10:48 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH RESEND v2 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily |
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On 11/09, Paul E. McKenney wrote: > > On Fri, Nov 09, 2012 at 04:46:56PM +0100, Oleg Nesterov wrote: > > Contrary, I am going to try to add some complications later, so that > > it can have more users. In particular, I think it can replace > > get_online_cpus/cpu_hotplug_begin, just we need > > percpu_down_write_but_dont_deadlock_with_recursive_readers(). > > I must confess that I am a bit concerned about possible scalability > bottlenecks in the current get_online_cpus(), so +1 from me on this one.
OK, thanks...
And btw percpu_down_write_but_dont_deadlock_with_recursive_readers() is trivial, just it needs down_write(rw_sem) "inside" wait_event(), not before. But I'm afraid I will never manage to write the comments ;)
static bool xxx(brw) { down_write(&brw->rw_sem); if (!atomic_read(&brw->slow_read_ctr)) return true;
up_write(&brw->rw_sem); return false; }
static void __percpu_down_write(struct percpu_rw_semaphore *brw, bool recursive_readers) { mutex_lock(&brw->writer_mutex);
synchronize_sched();
atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
if (recursive_readers) { wait_event(brw->write_waitq, xxx(brw)); } else { down_write(&brw->rw_sem);
wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr)); } }
Of course, cpu.c still needs .active_writer to allow get_online_cpus() under cpu_hotplug_begin(), but this is simple.
But first we should do other changes, I think. IMHO we should not do synchronize_sched() under mutex_lock() and this will add (a bit) more complications. We will see.
Oleg.
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