Re: Locking comment on shrink_caches()

[David S. Miller]

   From: Marcelo Tosatti <[e-mail]>
   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
[e-mail]