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Message-ID: <87lgevn0ss.fsf@linutronix.de>
Date: Tue, 13 Mar 2018 22:05:55 +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())
Hi Al,
1 minor issue on the new shrink_lock_dentry()...
> From 121a8e0834862d2c5d88c95f8e6bc8984f195abf Mon Sep 17 00:00:00 2001
> From: Al Viro <viro@...iv.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@...iv.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
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