linux-kernel - Re: [PATCH 4/4] fs/dcache: Avoid the try_lock loops in dentry

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Date:   Fri, 16 Feb 2018 10:03:15 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] fs/dcache: Avoid the try_lock loops in dentry_kill()

On Fri, Feb 16, 2018 at 7:09 AM, John Ogness <john.ogness@...utronix.de> wrote:
> dentry_kill() holds dentry->d_lock and needs to acquire both
> dentry->d_inode->i_lock and dentry->d_parent->d_lock. This cannot be
> done with spin_lock() operations because it's the reverse of the
> regular lock order. To avoid ABBA deadlocks it is done with two
> trylock loops.
>
> Trylock loops are problematic in two scenarios:

I don't mind this patch series per se (although I would really like Al
to ack it), but this particular patch I hate.

Why?

> Avoid the trylock loops by using dentry_lock_inode() and lock_parent()
> which take the locks in the appropriate order. As both functions drop
> dentry->lock briefly, this requires rechecking of the dentry content
> as it might have changed after dropping the lock.

I think the trylock should be done first, and then you don't need that
recheck for the common case.

I realize that the recheck itself isn't expensive, but it's mostly
about the code flow and the comment:

> +                * Recheck refcount as it might have been incremented while
> +                * d_lock was dropped.

the thing is, 99.9% of the time the d_lock wasn't dropped, so that
"while d_lock was dropped" comment is misleading.

Re-organizing it to do the trylock fastpath explicitly here and not
bothering with the re-check etc crid for the common case is the rioght
thing to do.

And the old code was actually organized exactly that way, with a

        if (inode && unlikely(!spin_trylock(&inode->i_lock)))
                goto failed;

at the top.

But instead of having that unlikely "failed" case do the complex
thing, you made the *normal* case do the complex thing.

So NAK on this.

It should be fairly trivial to fix, and make the "failed" thing do it right.

             Linus