linux-kernel - Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry

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Message-ID: <87zi3bn1on.fsf@linutronix.de>
Date:   Tue, 13 Mar 2018 21:46:48 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        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>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())

On 2018-03-12, Al Viro <viro@...IV.linux.org.uk> wrote:
>> If someone else has grabbed a reference, it shouldn't be added to the
>> lru list. Only decremented.
>> 
>> if (entry->d_lockref.count == 1)
>
> Nah, better handle that in retain_dentry() itself.  See updated
> #work.dcache.
>
> +	if (unlikely(dentry->d_lockref.count != 1)) {
> +		dentry->d_lockref.count--;
> +	} else if (likely(!retain_dentry(dentry))) {
> +		__dentry_kill(dentry);
> +		return parent;
> +	}

Although the updated version is correct (and saves on lines of code), I
find putting the deref and lru_add code in the "true" case of
retain_dentry() to be pretty tricky. IMHO the code is easier to
understand if it looks like this:

	if (unlikely(dentry->d_lockref.count != 1)) {
		dentry->d_lockref.count--;
	} else if (likely(!retain_dentry(dentry))) {
		__dentry_kill(dentry);
		return parent;
	} else {
		dentry->d_lockref.count--;
		dentry_lru_add(dentry);
	}

This is what your version is doing, but that final else is hiding in the
retain_dentry() "true" case.

My suggestion is to revert 7479f57fecd2a4837b5c79ce1cf0dcf284db54be (and
then fixup dput() to deref before calling dentry_lru_add()).

> FWIW, there's another trylock loop on dentries - one in
> autofs get_next_positive_dentry().  Any plans re dealing
> with that one?

I will need to dig into it a bit deeper (I am unfamiliar with autofs),
but it looks like it is trying to do basically the same thing as the
ascend loop in d_walk().

> I'd spent the last couple of weeks (when not being too sick
> for any work) going through dcache.c and related code; hopefully
> this time I will get the documentation into postable shape ;-/

Thank you for all your help in getting these changes cleaned and
correctly implemented so quickly.

I've reviewed your latest trylock loop removal patches and found only 1
minor issue. I'll post that separately.

John Ogness