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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