Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())

[Al Viro]

On Tue, Mar 13, 2018 at 10:05:55PM +0100, John Ogness wrote:

> > +   rcu_read_lock();                /* to protect parent */
> > +   spin_unlock(&dentry->d_lock);
> > +   parent = READ_ONCE(dentry->d_parent);
> 
> The preceeding line should be removed. We already have a "parent" from
> before we did the most recent trylock().

Nope.  We have parent, yes, but it had been fetched outside of rcu_read_lock().
So the object it used to point to might have been already freed and we
can't do this:

> > +   spin_lock(&parent->d_lock);

To get rid of that reread we'd need to do this:
        rcu_read_lock();
        parent = dentry->d_parent;
        if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock))) {
                rcu_read_unlock();
                return true;
        }
        spin_unlock(&dentry->d_lock);
        spin_lock(&parent->d_lock);
        if (unlikely(parent != dentry->d_parent)) {
                ....

Come to think of that, it might make sense to lift rcu_read_lock() all the
way out of that sucker.  Objections?  Below is the incremental I'd fold into
that commit:

diff --git a/fs/dcache.c b/fs/dcache.c
index f0e73c93182b..0d1dac750c0a 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1000,7 +1000,6 @@ static bool shrink_lock_dentry(struct dentry *dentry)
 
        inode = dentry->d_inode;
        if (inode && unlikely(!spin_trylock(&inode->i_lock))) {
-               rcu_read_lock();        /* to protect inode */
                spin_unlock(&dentry->d_lock);
                spin_lock(&inode->i_lock);
                spin_lock(&dentry->d_lock);
@@ -1009,16 +1008,14 @@ static bool shrink_lock_dentry(struct dentry *dentry)
                /* changed inode means that somebody had grabbed it */
                if (unlikely(inode != dentry->d_inode))
                        goto out;
-               rcu_read_unlock();
        }
 
        parent = dentry->d_parent;
+       /* parent will stay allocated until we drop rcu_read_lock */
        if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock)))
                return true;
 
-       rcu_read_lock();                /* to protect parent */
        spin_unlock(&dentry->d_lock);
-       parent = READ_ONCE(dentry->d_parent);
        spin_lock(&parent->d_lock);
        if (unlikely(parent != dentry->d_parent)) {
                spin_unlock(&parent->d_lock);
@@ -1026,14 +1023,11 @@ static bool shrink_lock_dentry(struct dentry *dentry)
                goto out;
        }
        spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
-       if (likely(!dentry->d_lockref.count)) {
-               rcu_read_unlock();
+       if (likely(!dentry->d_lockref.count))
                return true;
-       }
        spin_unlock(&parent->d_lock);
 out:
        spin_unlock(&inode->i_lock);
-       rcu_read_unlock();
        return false;
 }
 
@@ -1044,8 +1038,10 @@ static void shrink_dentry_list(struct list_head *list)
 
                dentry = list_entry(list->prev, struct dentry, d_lru);
                spin_lock(&dentry->d_lock);
+               rcu_read_lock();
                if (!shrink_lock_dentry(dentry)) {
                        bool can_free = false;
+                       rcu_read_unlock();
                        d_shrink_del(dentry);
                        if (dentry->d_lockref.count < 0)
                                can_free = dentry->d_flags & DCACHE_MAY_FREE;
@@ -1054,6 +1050,7 @@ static void shrink_dentry_list(struct list_head *list)
                                dentry_free(dentry);
                        continue;
                }
+               rcu_read_unlock();
                d_shrink_del(dentry);
                parent = dentry->d_parent;
                __dentry_kill(dentry);