LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
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>,
	Eric Biederman <ebiederm@xmission.com>
Subject: Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())
Date: Tue, 13 Mar 2018 22:05:55 +0100	[thread overview]
Message-ID: <87lgevn0ss.fsf@linutronix.de> (raw)
In-Reply-To: <87zi3bn1on.fsf@linutronix.de> (John Ogness's message of "Tue, 13 Mar 2018 21:46:48 +0100")

Hi Al,

1 minor issue on the new shrink_lock_dentry()...

> From 121a8e0834862d2c5d88c95f8e6bc8984f195abf Mon Sep 17 00:00:00 2001
> From: Al Viro <viro@zeniv.linux.org.uk>
> Date: Fri, 23 Feb 2018 21:54:18 -0500
> Subject: [PATCH] get rid of trylock loop in locking dentries on shrink
>  list
>
> In case of trylock failure don't re-add to the list - drop the locks
> and carefully get them in the right order.  For shrink_dentry_list(),
> somebody having grabbed a reference to dentry means that we can
> kick it off-list, so if we find dentry being modified under us we
> don't need to play silly buggers with retries anyway - off the list
> it is.
>
> The locking logics taken out into a helper of its own; lock_parent()
> is no longer used for dentries that can be killed under us.
>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>  fs/dcache.c | 103 ++++++++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 66 insertions(+), 37 deletions(-)
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 1684b6b..58097fd 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -974,56 +974,85 @@ void d_prune_aliases(struct inode *inode)
>  }
>  EXPORT_SYMBOL(d_prune_aliases);
>  
> -static void shrink_dentry_list(struct list_head *list)
> +/*
> + * Lock a dentry from shrink list.
> + * Note that dentry is *not* protected from concurrent dentry_kill(),
> + * d_delete(), etc.  It is protected from freeing (by the fact of
> + * being on a shrink list), but everything else is fair game.
> + * Return false if dentry has been disrupted or grabbed, leaving
> + * the caller to kick it off-list.  Otherwise, return true and have
> + * that dentry's inode and parent both locked.
> + */
> +static bool shrink_lock_dentry(struct dentry *dentry)
>  {
> -	struct dentry *dentry, *parent;
> +	struct inode *inode;
> +	struct dentry *parent;
>  
> -	while (!list_empty(list)) {
> -		struct inode *inode;
> -		dentry = list_entry(list->prev, struct dentry, d_lru);
> +	if (dentry->d_lockref.count)
> +		return false;
> +
> +	inode = dentry->d_inode;
> +	if (inode && unlikely(!spin_trylock(&inode->i_lock))) {
> +		rcu_read_lock();	/* to protect inode */
> +		spin_unlock(&dentry->d_lock);
> +		spin_lock(&inode->i_lock);
>  		spin_lock(&dentry->d_lock);
> -		parent = lock_parent(dentry);
> +		if (unlikely(dentry->d_lockref.count))
> +			goto out;
> +		/* changed inode means that somebody had grabbed it */
> +		if (unlikely(inode != dentry->d_inode))
> +			goto out;
> +		rcu_read_unlock();
> +	}
>  
> -		/*
> -		 * The dispose list is isolated and dentries are not accounted
> -		 * to the LRU here, so we can simply remove it from the list
> -		 * here regardless of whether it is referenced or not.
> -		 */
> -		d_shrink_del(dentry);
> +	parent = dentry->d_parent;
> +	if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock)))
> +		return true;
>  
> -		/*
> -		 * We found an inuse dentry which was not removed from
> -		 * the LRU because of laziness during lookup. Do not free it.
> -		 */
> -		if (dentry->d_lockref.count > 0) {
> -			spin_unlock(&dentry->d_lock);
> -			if (parent)
> -				spin_unlock(&parent->d_lock);
> -			continue;
> -		}
> +	rcu_read_lock();		/* to protect parent */
> +	spin_unlock(&dentry->d_lock);
> +	parent = READ_ONCE(dentry->d_parent);

The preceeding line should be removed. We already have a "parent" from
before we did the most recent trylock().

> +	spin_lock(&parent->d_lock);
> +	if (unlikely(parent != dentry->d_parent)) {
> +		spin_unlock(&parent->d_lock);
> +		spin_lock(&dentry->d_lock);
> +		goto out;
> +	}
> +	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
> +	if (likely(!dentry->d_lockref.count)) {
> +		rcu_read_unlock();
> +		return true;
> +	}
> +	spin_unlock(&parent->d_lock);
> +out:
> +	spin_unlock(&inode->i_lock);
> +	rcu_read_unlock();
> +	return false;
> +}
>  
> +static void shrink_dentry_list(struct list_head *list)
> +{
> +	while (!list_empty(list)) {
> +		struct dentry *dentry, *parent;
> +		struct inode *inode;
>  
> -		if (unlikely(dentry->d_flags & DCACHE_DENTRY_KILLED)) {
> -			bool can_free = dentry->d_flags & DCACHE_MAY_FREE;
> +		dentry = list_entry(list->prev, struct dentry, d_lru);
> +		spin_lock(&dentry->d_lock);
> +		if (!shrink_lock_dentry(dentry)) {
> +			bool can_free = false;
> +			d_shrink_del(dentry);
> +			if (dentry->d_lockref.count < 0)
> +				can_free = dentry->d_flags & DCACHE_MAY_FREE;
>  			spin_unlock(&dentry->d_lock);
> -			if (parent)
> -				spin_unlock(&parent->d_lock);
>  			if (can_free)
>  				dentry_free(dentry);
>  			continue;
>  		}
> -
> -		inode = dentry->d_inode;
> -		if (inode && unlikely(!spin_trylock(&inode->i_lock))) {
> -			d_shrink_add(dentry, list);
> -			spin_unlock(&dentry->d_lock);
> -			if (parent)
> -				spin_unlock(&parent->d_lock);
> -			continue;
> -		}
> -
> +		d_shrink_del(dentry);
> +		parent = dentry->d_parent;
>  		__dentry_kill(dentry);
> -
> +		if (parent == dentry)
> +			continue;
>  		/*
>  		 * We need to prune ancestors too. This is necessary to prevent
>  		 * quadratic behavior of shrink_dcache_parent(), but is also

John Ogness

  reply	other threads:[~2018-03-13 21:06 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                     ` dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list()) John Ogness
2018-03-12 19:13                       ` 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 [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=87lgevn0ss.fsf@linutronix.de \
    --to=john.ogness@linutronix.de \
    --cc=bigeasy@linutronix.de \
    --cc=ebiederm@xmission.com \
    --cc=hch@lst.de \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=peterz@infradead.org \
    --cc=tglx@linutronix.de \
    --cc=torvalds@linux-foundation.org \
    --cc=viro@ZenIV.linux.org.uk \
    --subject='Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).