Re: [PATCH v2 4/6] fs/dcache: Avoid the try_lock loops in dentry_kill()

[Al Viro]

On Fri, Feb 23, 2018 at 02:22:43AM +0000, Al Viro wrote:

> No.  This is completely wrong.  If somebody else has found the sucker
> while we dropped the lock and even got around to playing with refcount,
> they might have done more than that.
> 
> In particular, they might have *dropped* their reference, after e.g.
> picking it as our inode's alias and rehashed the fucker.  Making
> our decision not to retain it no longer valid.  And your code will
> not notice that.

PS: I really wonder if we should treat the failure to trylock ->i_lock
and parent's ->d_lock at that point (we are already off the fast path
here) as
        * drop all spinlocks we'd got
        * grab ->i_lock
        * grab ->d_lock
        * lock_parent()
        * act as if fast_dput() has returned 0, only remember to drop ->i_lock
and unlock parent before dropping ->d_lock if we decide to keep it.

IOW, add

static inline bool retain_dentry(struct dentry *dentry)
{
        WARN_ON(d_in_lookup(dentry));

        /* Unreachable? Get rid of it */
        if (unlikely(d_unhashed(dentry)))
                return false;

        if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
                return false;

        if (unlikely(dentry->d_flags & DCACHE_OP_DELETE)) {
                if (dentry->d_op->d_delete(dentry))
                        return false;
        }

        dentry_lru_add(dentry);
        dentry->d_lockref.count--;
        return true;
}

then have dput() do
{
        if (unlikely(!dentry))
                return;
repeat:
        might_sleep();

        rcu_read_lock();
        if (likely(fast_dput(dentry))) {
                rcu_read_unlock();
                return;
        }

        /* Slow case: now with the dentry lock held */
        rcu_read_unlock();
        if (likely(retain_dentry(dentry))) {
                spin_unlock(&dentry->d_lock);
                return;
        }
        dentry = dentry_kill(dentry);
        if (dentry)
                goto repeat;
}

with dentry_kill() being pretty much as it is now, except that
it would be ending on

failed:
        spin_unlock(&dentry->d_lock);
        spin_lock(&inode->i_lock);
        spin_lock(&dentry->d_lock);
        parent = lock_parent(dentry);
        /* retain_dentry() needs ->count == 1 already checked)
        if (dentry->d_lockref.count == 1 && !retain_dentry(dentry)) {
                __dentry_kill(dentry);
                return parent;
        }
        /* we are keeping it, after all */
        if (inode)
                spin_unlock(&inode->i_lock);
        spin_unlock(&dentry->d_lock);
        if (parent)
                spin_unlock(&parent->d_lock);
        return NULL;
}