linux-kernel - Re: [PATCH v2 5/6] fs/dcache: Avoid a try_lock loop in shrink_dentry

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Date:   Fri, 23 Feb 2018 03:48:27 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-fsdevel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/6] fs/dcache: Avoid a try_lock loop in
 shrink_dentry_list()

On Fri, Feb 23, 2018 at 12:50:24AM +0100, John Ogness wrote:
> -		while (dentry && !lockref_put_or_lock(&dentry->d_lockref)) {
> -			parent = lock_parent(dentry);
> -			if (dentry->d_lockref.count != 1) {
> -				dentry->d_lockref.count--;
> -				spin_unlock(&dentry->d_lock);
> -				if (parent)
> -					spin_unlock(&parent->d_lock);
> -				break;
> -			}
> -			inode = dentry->d_inode;	/* can't be NULL */
> -			if (unlikely(!spin_trylock(&inode->i_lock))) {
> -				spin_unlock(&dentry->d_lock);
> -				if (parent)
> -					spin_unlock(&parent->d_lock);
> -				cpu_relax();
> -				continue;
> -			}
> -			__dentry_kill(dentry);
> -			dentry = parent;
> -		}
> +		while (dentry && !lockref_put_or_lock(&dentry->d_lockref))
> +			dentry = dentry_kill(dentry);

Hmm...  OK, that's interesting.  I agree that it looks similar to dentry_kill()
loop, with one exception - here we are aggressively pruning the branch.  None
of the "do we want to retain that sucker" stuff here.  It doesn't matter for
most of the callers, with one exception: prune_dcache_sb().  OTOH, there it
just might be the right thing to do anyway - after all, it matters only if
somebody has grabbed and dropped the sucker while we'd been trying to do
lock_parent().  Had we lost the race with their dput(), we would've left
the damn thing alone, and we are called from a memory shrinker, so we'll get
called again if needed.