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    SubjectRe: [PATCH 04/19] fs: Implement lazy LRU updates for inodes.
    On Sat, Oct 16, 2010 at 12:59:30PM -0400, Christoph Hellwig wrote:
    > On Sat, Oct 16, 2010 at 08:29:16PM +1100, Nick Piggin wrote:
    > > On Sat, Oct 16, 2010 at 07:13:58PM +1100, Dave Chinner wrote:
    > > > @@ -502,11 +527,15 @@ static void prune_icache(int nr_to_scan)
    > > > iput(inode);
    > > > spin_lock(&inode_lock);
    > > >
    > > > - if (inode != list_entry(inode_unused.next,
    > > > - struct inode, i_list))
    > > > - continue; /* wrong inode or list_empty */
    > > > - if (!can_unuse(inode))
    > > > + /*
    > > > + * if we can't reclaim this inode immediately, give it
    > > > + * another pass through the free list so we don't spin
    > > > + * on it.
    > > > + */
    > > > + if (!can_unuse(inode)) {
    > > > + list_move(&inode->i_list, &inode_unused);
    > > > continue;
    > > > + }
    > > > }
    > > > list_move(&inode->i_list, &freeable);
    > > > WARN_ON(inode->i_state & I_NEW);
    > >
    > > This is a bug, actually 2 bugs, which is why I omitted it in the version
    > > you picked up. I agree we want the optimisation though, so I've added it
    > > back in my tree.
    > >
    > > After you iput() and then re take the inode lock, you can't reference
    > > the inode because you don't know what happened to it. You need to keep
    > > that pointer check to verify it is still there.
    >
    > I don't think the pointer check will work either. By the time we retake
    > the lru lock the inode might already have been reaped through a call
    > to invalidate_inodes. There's no way we can do anything with it after

    I don't think you're right. If we re take inode_lock, ensure it is on
    the LRU, and call the can_unuse checks, there is no more problem than
    the regular loop taking items from the LRU, AFAIKS.

    > iput. What we could do is using variant of can_unuse to decide to move
    > the inode to the front of the lru before doing the iput. That way
    > we'll get to it next after retaking the lru lock if it's still there.

    This might actually be the better approach anyway (even for upstream)
    -- it means we don't have to worry about the "check head element"
    heuristic of the LRU check which could get false negatives if there is
    a lot of concurrency on the LRU.




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