Re: [PATCH v5 04/45] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks

On 02/11, Srivatsa S. Bhat wrote:

On 02/09/2013 04:40 AM, Paul E. McKenney wrote:

quoted

quoted

+static void announce_writer_inactive(struct percpu_rwlock *pcpu_rwlock)
+{
+	unsigned int cpu;
+
+	drop_writer_signal(pcpu_rwlock, smp_processor_id());

Why do we drop ourselves twice?  More to the point, why is it important to
drop ourselves first?

I don't see where we are dropping ourselves twice. Note that we are no longer
in the cpu_online_mask, so the 'for' loop below won't include us. So we need
to manually drop ourselves. It doesn't matter whether we drop ourselves first
or later.

Yes, but this just reflects its usage in cpu-hotplug. cpu goes away under
_write_lock.

Perhaps _write_lock/unlock shoud use for_each_possible_cpu() instead?

Hmm... I think this makes sense anyway. Otherwise, in theory,
percpu_write_lock(random_non_hotplug_lock) can race with cpu_up?

Oleg.