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 04:08:14 +0000 [thread overview]
Message-ID: <20180223040814.GA30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20180223035814.GZ30522@ZenIV.linux.org.uk>
On Fri, Feb 23, 2018 at 03:58:14AM +0000, Al Viro wrote:
> On Fri, Feb 23, 2018 at 12:50:25AM +0100, John Ogness wrote:
>
> > Avoid the trylock loop by using dentry_kill(). When killing dentries
> > from the dispose list, it is very similar to killing a dentry in
> > dput(). The difference is that dput() expects to be the last user of
> > the dentry (refcount=1) and will deref whereas shrink_dentry_list()
> > expects there to be no user (refcount=0). In order to handle both
> > situations with the same code, move the deref code from dentry_kill()
> > into a new wrapper function dentry_put_kill(), which can be used
> > by previous dentry_kill() users. Then shrink_dentry_list() can use
> > the dentry_kill() to cleanup the dispose list.
> >
> > This also has the benefit that the locking order is now the same.
> > First the inode is locked, then the parent.
>
> Current code moves the sucker to the end of list in that case; I'm not
> at all sure that what you are doing will improve the situation at all...
>
> You *still* have a trylock loop there - only it keeps banging at the
> same dentry instead of going through the rest first...
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.
next prev parent reply other threads:[~2018-02-23 4:08 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 [this message]
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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