Re: Locking comment on shrink_caches()

David S. Miller
2001-09-25T19:58:03Z

From: Marcelo Tosatti <marcelo@conectiva.com.br>
Date: Tue, 25 Sep 2001 14:49:40 -0300 (BRT)

Do you really need to do this ?

if (unlikely(!spin_trylock(&pagecache_lock))) {
/* we hold the page lock so the page cannot go away from under us */
spin_unlock(&pagemap_lru_lock);

spin_lock(&pagecache_lock);
spin_lock(&pagemap_lru_lock);
}

Have you actually seen bad hold times of pagecache_lock by
shrink_caches() ?

Marcelo, this is needed because of the spin lock ordering rules.
The pagecache_lock must be obtained before the pagemap_lru_lock
or else deadlock is possible. The spin_trylock is an optimization.

Franks a lot,
David S. Miller
davem@redhat.com