On Thu, Jun 26, 2008 at 11:49 AM, Max Krasnyansky <maxk@qualcomm.com> wrote:
No, I think it is a problem.
When a CPU goes down, the hotplug code holds cpu_hotplug.lock and
calls cleanup_workqueue_thread() which waits for any running work on
that thread to finish.
So if the workqueue thread running on that CPU calls get_online_cpus()
while the hotplug thread is waiting for it, it will deadlock. (It's
not clear to me how lockdep figured this out - I guess it's something
to do with the *_acquire() annotations that tell lockdep to treat the
workqueue structure as a pseudo-lock?)
I guess the fix for that would be to have a non-workqueue thread to
handle the async domain rebuilding, which isn't tied to a particular
cpu the way workqueue threads are.
OK, so the problem is that since cpu_hotplug contains a hand-rolled
rwsem, lockdep is going to spot false deadlocks.
Is there any reason not to replace cpu_hotplug.lock and
cpu_hotplug.refcount with an rw_semaphore, and have the following:
void get_online_cpus(void)
{
might_sleep();
if (cpu_hotplug.active_writer == current)
return;
down_read(&cpu_hotplug.lock);
}
void put_online_cpus(void)
{
if (cpu_hotplug.active_writer == current)
return;
up_read(&cpu_hotplug.lock);
}
static void cpu_hotplug_begin(void)
{
down_write(&cpu_hotplug.lock);
cpu_hotplug.active_writer = current;
}
static void cpu_hotplug_done(void)
{
cpu_hotplug.active_writer = NULL;
up_write(&cpu_hotplug.lock);
}
I think that combined with moving the async rebuild_sched_domains to a
separate thread should solve the problem, but I'm wondering why
cpu_hotplug.lock was implemented this way in the first place.
Paul
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