Re: Locking comment on shrink_caches()

[Marcelo Tosatti]

On Tue, 25 Sep 2001, David S. Miller wrote:

>    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.

Not, it is not.

We can simply lock the pagecachelock and the pagemap_lru_lock at the
beginning of the cleaning function. page_launder() use to do that.

Thats why I asked Andrea if there was long hold times by shrink_caches().