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

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Date:   Sat, 17 Feb 2018 00:05:35 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 2018-02-16, Linus Torvalds <torvalds@...ux-foundation.org> 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.

dentry_lock_inode() uses the same semantics as lock_parent(). The caller
does not know if the trylock succeeded. Any caller using lock_parent()
must "recheck in case I dropped", just as with dentry_lock_inode(). This
is what you have pointed out.

> 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?

dentry_kill() calls both dentry_lock_inode() and lock_parent() in the
common case. So by changing the semantics of lock_parent(), I am
removing two "recheck in case I dropped" in the common case rather than
just the one you pointed out.

John Ogness