LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
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 6/6] fs/dcache: Avoid remaining try_lock loop in shrink_dentry_list()
Date: Fri, 23 Feb 2018 15:09:28 +0000	[thread overview]
Message-ID: <20180223150928.GC30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <87h8q7erlo.fsf@linutronix.de>

On Fri, Feb 23, 2018 at 02:57:23PM +0100, John Ogness wrote:

> > Actually, it's even worse - _here_ you are dealing with something that
> > really can change inode under you.  This is one and only case where we
> > are kicking out a zero-refcount dentry without having already held
> > ->i_lock.  At the very least, it's bloody different from regular
> > dentry_kill().  In this case, dentry itself is protected from freeing
> > by being on the shrink list - that's what makes __dentry_kill() to
> > leave the sucker allocated.  We are not holding references, it is
> > hashed and anybody could come, pick it, d_delete() it, etc.
> 
> Yes, and that is why the new dentry_lock_inode() and dentry_kill()
> functions react to any changes in refcount and check for inode
> changes. Obviously for d_delete() the helper functions are checking way
> more than they need to. But if we've missed the trylock optimization
> we're already in the unlikely case, so the extra checks _may_ be
> acceptable in order to have simplified code. As Linus already pointed
> out, the cost of spinning will likely overshadow the cost of a few
> compares.

It's not that you are checking extra things - you are checking the wrong
things.  "Refcount has returned to original" is useless.

> Do you recommend I avoid consolidating the 4 trylock loops into the same
> set of helper functions and instead handle them all separately (as is
> the case in mainline)?
> 
> Or maybe the problem is how my patchset is assembling the final
> result. If patch 3 and 4 were refined to address your concerns about
> them but then by the end of the 6th patch we still end up where we are
> now, is that something that is palatable?

No.  The place where you end up with dput() is flat-out wrong.

> IOW, do the patches only need (possibly a lot of) refinement or do you
> consider this approach fundamentally flawed?

You are conflating the "we have a reference" cases with this one, and
they are very different.  Note, BTW, that had we raced with somebody
else grabbing a reference, we would've quietly dropped dentry from
the shrink list; what if we do the following: just after checking that
refcount is not positive, do
	inode = dentry->d_inode;
	if unlikely(inode && !spin_trylock...)
		rcu_read_lock
		drop ->d_lock
		grab inode->i_lock
		grab ->d_lock
		if unlikely(dentry->d_inode != inode)
			drop inode->i_lock
			rcu_read_unlock
			if !killed
				drop ->d_lock
				drop parent's ->d_lock
				continue;
		else
			rcu_read_unlock
*before* going into
                if (unlikely(dentry->d_flags & DCACHE_DENTRY_KILLED)) {
                        bool can_free = dentry->d_flags & DCACHE_MAY_FREE;
                        spin_unlock(&dentry->d_lock);
			...
part?

  reply	other threads:[~2018-02-23 15:09 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 [this message]
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

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=20180223150928.GC30522@ZenIV.linux.org.uk \
    --to=viro@zeniv.linux.org.uk \
    --cc=bigeasy@linutronix.de \
    --cc=hch@lst.de \
    --cc=john.ogness@linutronix.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 \
    --subject='Re: [PATCH v2 6/6] fs/dcache: Avoid remaining try_lock loop in 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).