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

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Date:   Fri, 16 Feb 2018 14:42:36 -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 2:32 PM, John Ogness <john.ogness@...utronix.de> wrote:
>
> lock_parent() already has the problem you are referring to. Callers are
> required to recheck the dentry contents and check the returned parent
> because they do not know if the trylock succeeded. See
> d_prune_aliases(), for example.

What are you talking about?

lock_parent() does the nice "spin_trylock succeeded" special case.

Yes, it will then do the "unlock dentry, do the parent first, then
re-check" too, and callers may need to worry about it.

But that's not what I'm complaining about in your patch. You remove
the simple case, and make dentry_kill() do the "recheck in case I
dropped" every single time.

It's the "turn a simple case into a complex case" that I absolutely detest.

The fact that there are _other_ complex cases doesn't make it any
better. The whole "but Bobby does it too" thing is not a defense.
Would you jump off a bridge just because your friend did it?

               Linus