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From: John Ogness <john.ogness@linutronix.de>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())
Date: Tue, 27 Feb 2018 06:16:28 +0100	[thread overview]
Message-ID: <87bmgbnhar.fsf_-_@linutronix.de> (raw)
In-Reply-To: <20180225073950.GI30522@ZenIV.linux.org.uk> (Al Viro's message of "Sun, 25 Feb 2018 07:40:05 +0000")

On 2018-02-25, Al Viro <viro@ZenIV.linux.org.uk> wrote:
> I'll play with cleaning up and reordering tomorrow after I get some
> sleep.  In the meanwhile, the current state of that set is at
> git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git #work.dcache

I have one comment on your new dentry_kill()...

> From e77db95d1ad189c76bcda7b90ac2225e98d776a2 Mon Sep 17 00:00:00 2001
> From: Al Viro <viro@zeniv.linux.org.uk>
> Date: Fri, 23 Feb 2018 21:25:42 -0500
> Subject: [PATCH] get rid of trylock loop around dentry_kill()
>
> In case when trylock in there fails, deal with it directly in
> dentry_kill().  Note that in cases when we drop and retake
> ->d_lock, we need to recheck whether to retain the dentry.
> Another thing is that dropping/retaking ->d_lock might have
> ended up with negative dentry turning into positive; that,
> of course, can happen only once...
>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>  fs/dcache.c | 34 +++++++++++++++++++++++++++-------
>  1 file changed, 27 insertions(+), 7 deletions(-)
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 449c1a5..c1f082d 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -657,23 +657,43 @@ static struct dentry *dentry_kill(struct dentry *dentry)
>  	struct dentry *parent = NULL;
>  
>  	if (inode && unlikely(!spin_trylock(&inode->i_lock)))
> -		goto failed;
> +		goto slow_positive;
>  
>  	if (!IS_ROOT(dentry)) {
>  		parent = dentry->d_parent;
>  		if (unlikely(!spin_trylock(&parent->d_lock))) {
> -			if (inode)
> -				spin_unlock(&inode->i_lock);
> -			goto failed;
> +			parent = __lock_parent(dentry);
> +			if (likely(inode || !dentry->d_inode))
> +				goto got_locks;
> +			/* negative that became positive */
> +			if (parent)
> +				spin_unlock(&parent->d_lock);
> +			inode = dentry->d_inode;
> +			goto slow_positive;
>  		}
>  	}
> -
>  	__dentry_kill(dentry);
>  	return parent;
>  
> -failed:
> +slow_positive:
> +	spin_unlock(&dentry->d_lock);
> +	spin_lock(&inode->i_lock);
> +	spin_lock(&dentry->d_lock);
> +	parent = lock_parent(dentry);
> +got_locks:
> +	if (likely(dentry->d_lockref.count == 1 && !retain_dentry(dentry))) {
> +		__dentry_kill(dentry);
> +		return parent;
> +	}
> +	/* we are keeping it, after all */
> +	if (inode)
> +		spin_unlock(&inode->i_lock);
> +	if (parent)
> +		spin_unlock(&parent->d_lock);

If someone else has grabbed a reference, it shouldn't be added to the
lru list. Only decremented.

if (entry->d_lockref.count == 1)

> +	dentry_lru_add(dentry);
> +	dentry->d_lockref.count--;
>  	spin_unlock(&dentry->d_lock);
> -	return dentry; /* try again with same dentry */
> +	return NULL;
>  }
>  
>  /*

  reply	other threads:[~2018-02-27  5:16 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-22 23:50 [PATCH v2 0/6] fs/dcache: avoid trylock loops John Ogness
2018-02-22 23:50 ` [PATCH v2 1/6] fs/dcache: Remove stale comment from dentry_kill() John Ogness
2018-02-22 23:50 ` [PATCH v2 2/6] fs/dcache: Move dentry_kill() below lock_parent() John Ogness
2018-02-22 23:50 ` [PATCH v2 3/6] fs/dcache: Avoid the try_lock loop in d_delete() John Ogness
2018-02-23  2:08   ` Al Viro
2018-02-22 23:50 ` [PATCH v2 4/6] fs/dcache: Avoid the try_lock loops in dentry_kill() John Ogness
2018-02-23  2:22   ` Al Viro
2018-02-23  3:12     ` Al Viro
2018-02-23  3:16       ` Al Viro
2018-02-23  5:46       ` Al Viro
2018-02-22 23:50 ` [PATCH v2 5/6] fs/dcache: Avoid a try_lock loop in shrink_dentry_list() John Ogness
2018-02-23  3:48   ` Al Viro
2018-02-22 23:50 ` [PATCH v2 6/6] fs/dcache: Avoid remaining " John Ogness
2018-02-23  3:58   ` Al Viro
2018-02-23  4:08     ` Al Viro
2018-02-23 13:57       ` John Ogness
2018-02-23 15:09         ` Al Viro
2018-02-23 17:42           ` Al Viro
2018-02-23 20:13             ` [BUG] lock_parent() breakage when used from shrink_dentry_list() (was Re: [PATCH v2 6/6] fs/dcache: Avoid remaining try_lock loop in shrink_dentry_list()) Al Viro
2018-02-23 21:35               ` Linus Torvalds
2018-02-24  0:22                 ` Al Viro
2018-02-25  7:40                   ` Al Viro
2018-02-27  5:16                     ` John Ogness [this message]
2018-03-12 19:13                       ` dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list()) Al Viro
2018-03-12 20:05                         ` Al Viro
2018-03-12 20:33                           ` Al Viro
2018-03-13  1:12                           ` NeilBrown
2018-04-28  0:10                             ` Al Viro
2018-03-12 20:23                         ` Eric W. Biederman
2018-03-12 20:39                           ` Al Viro
2018-03-12 23:28                             ` Eric W. Biederman
2018-03-12 23:52                               ` Eric W. Biederman
2018-03-13  0:37                                 ` Al Viro
2018-03-13  0:50                                   ` Al Viro
2018-03-13  4:02                                     ` Eric W. Biederman
2018-03-14 23:20                                     ` [PATCH] fs: Teach path_connected to handle nfs filesystems with multiple roots Eric W. Biederman
2018-03-15 22:34                                       ` Al Viro
2018-03-13  0:36                               ` dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list()) Al Viro
2018-03-12 22:14                         ` Thomas Gleixner
2018-03-13 20:46                         ` John Ogness
2018-03-13 21:05                           ` John Ogness
2018-03-13 23:59                             ` Al Viro
2018-03-14  2:58                               ` Matthew Wilcox
2018-03-14  8:18                               ` John Ogness
2018-03-02  9:04                     ` [BUG] lock_parent() breakage when used from shrink_dentry_list() (was Re: [PATCH v2 6/6] fs/dcache: Avoid remaining try_lock loop in shrink_dentry_list()) Sebastian Andrzej Siewior
2018-02-23  0:59 ` [PATCH v2 0/6] fs/dcache: avoid trylock loops Linus Torvalds

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