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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 5/6] fs/dcache: Avoid a try_lock loop in shrink_dentry_list()
Date: Fri, 23 Feb 2018 03:48:27 +0000	[thread overview]
Message-ID: <20180223034827.GY30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20180222235025.28662-6-john.ogness@linutronix.de>

On Fri, Feb 23, 2018 at 12:50:24AM +0100, John Ogness wrote:
> -		while (dentry && !lockref_put_or_lock(&dentry->d_lockref)) {
> -			parent = lock_parent(dentry);
> -			if (dentry->d_lockref.count != 1) {
> -				dentry->d_lockref.count--;
> -				spin_unlock(&dentry->d_lock);
> -				if (parent)
> -					spin_unlock(&parent->d_lock);
> -				break;
> -			}
> -			inode = dentry->d_inode;	/* can't be NULL */
> -			if (unlikely(!spin_trylock(&inode->i_lock))) {
> -				spin_unlock(&dentry->d_lock);
> -				if (parent)
> -					spin_unlock(&parent->d_lock);
> -				cpu_relax();
> -				continue;
> -			}
> -			__dentry_kill(dentry);
> -			dentry = parent;
> -		}
> +		while (dentry && !lockref_put_or_lock(&dentry->d_lockref))
> +			dentry = dentry_kill(dentry);

Hmm...  OK, that's interesting.  I agree that it looks similar to dentry_kill()
loop, with one exception - here we are aggressively pruning the branch.  None
of the "do we want to retain that sucker" stuff here.  It doesn't matter for
most of the callers, with one exception: prune_dcache_sb().  OTOH, there it
just might be the right thing to do anyway - after all, it matters only if
somebody has grabbed and dropped the sucker while we'd been trying to do
lock_parent().  Had we lost the race with their dput(), we would've left
the damn thing alone, and we are called from a memory shrinker, so we'll get
called again if needed.

  reply	other threads:[~2018-02-23  3:48 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 [this message]
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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