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From: Al Viro <viro@ZenIV.linux.org.uk>
To: John Ogness <john.ogness@linutronix.de>
Cc: linux-fsdevel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/6] fs/dcache: Avoid the try_lock loops in dentry_kill()
Date: Fri, 23 Feb 2018 02:22:43 +0000	[thread overview]
Message-ID: <20180223022242.GV30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20180222235025.28662-5-john.ogness@linutronix.de>

On Fri, Feb 23, 2018 at 12:50:23AM +0100, John Ogness wrote:
>  static struct dentry *dentry_kill(struct dentry *dentry)
>  	__releases(dentry->d_lock)
>  {
> -	struct inode *inode = dentry->d_inode;
> -	struct dentry *parent = NULL;
> +	int saved_count = dentry->d_lockref.count;

	Umm...  How can that be not 1?  After all, fast_dput() should
never return false without ->d_lock being held *and* ->d_count being
equal to 1.

> +	/*
> +	 * d_inode might have changed if d_lock was temporarily
> +	 * dropped. If it changed it is necessary to start over
> +	 * because a wrong inode (or no inode) lock is held.
> +	 */

If it might have changed, we are fucked.

> +out_ref_changed:
> +	/*
> +	 * The refcount was incremented while dentry->d_lock was dropped.
> +	 * Just decrement the refcount, unlock, and tell the caller to
> +	 * stop the directory walk.
> +	 */
> +	if (!WARN_ON(dentry->d_lockref.count < 1))
> +		dentry->d_lockref.count--;
> +
>  	spin_unlock(&dentry->d_lock);
> -	return dentry; /* try again with same dentry */
> +
> +	return NULL;

No.  This is completely wrong.  If somebody else has found the sucker
while we dropped the lock and even got around to playing with refcount,
they might have done more than that.

In particular, they might have *dropped* their reference, after e.g.
picking it as our inode's alias and rehashed the fucker.  Making
our decision not to retain it no longer valid.  And your code will
not notice that.

  reply	other threads:[~2018-02-23  2:22 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 [this message]
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
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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