Change uma_mtx to rwlock

[John Baldwin]

On Saturday, September 27, 2014 07:59:47 PM Bryan Venteicher wrote:
> Hi,
> 
> I'd appreciate some comments attached patch that changes the uma_mtx to a
> rwlock.
> 
> At $JOB, we have machines with ~400GB RAM, with much of that being
> allocated through UMA zones. We've observed that timeouts were sometimes
> unexpectedly delayed by a half second or more. We tracked one of the
> reasons for this down to when the page daemon was running, calling
> uma_reclaim() -> zone_foreach(). zone_foreach() holds the uma_mtx while
> zone_drain()'ing each zone. If uma_timeout() fires, it will block on the
> uma_mtx when it tries to zone_timeout() each zone.

The only nit I see is in zone_drain_wait().  It would be nice to not need the 
hack of checking for a read or write lock and just require the one it actually 
needs depending on the callers.

However, checking the code in HEAD, this appears to just be broken.  
Specifically, zone_drain_wait() is called in two places:

void
zone_drain(uma_zone_t zone)
{

	zone_drain_wait(zone, M_NOWAIT);
}

...


static void
zone_dtor(void *arg, int size, void *udata)
{
	...
	mtx_lock(&uma_mtx);
	LIST_REMOVE(zone, uz_link);
	mtx_unlock(&uma_mtx);
	/*
	 * XXX there are some races here where
	 * the zone can be drained but zone lock
	 * released and then refilled before we
	 * remove it... we dont care for now
	 */
	zone_drain_wait(zone, M_WAITOK);
	...
}

Neither one calls it with the uma_mtx locked!  This appears to have been 
broken since that function was introduced in r187681.

I think it might be best to first remove the unlock/lock of uma_mtx from 
zone_drain_wait() (so it can be MFC'd).  That then simplifies that one part of 
your patch (which I think is otherwise fine).

-- 
John Baldwin