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From: Al Viro <viro@ZenIV.linux.org.uk>
To: NeilBrown <neilb@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Eric Biederman <ebiederm@xmission.com>
Subject: Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())
Date: Mon, 12 Mar 2018 20:05:41 +0000	[thread overview]
Message-ID: <20180312200540.GO30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20180312191351.GN30522@ZenIV.linux.org.uk>

On Mon, Mar 12, 2018 at 07:13:51PM +0000, Al Viro wrote:

> There's an unpleasant area around the ->s_root vs. NFS.  There's
> code that makes assumptions about ->s_root that are simply not true
> for NFS.  Is path_connected() correct wrt NFS multiple imports from
> the same server?  Ditto for mnt_already_visible() (that one might
> be mitigated at the moment, but probably won't last).  Eric, am
> I missing something subtle in there?

BTW, another thing spotted while trying to document the stuff:
Neil's "VFS: don't keep disconnected dentries on d_anon" has a subtle
side effect I hadn't spotted when applying.  Namely, for
DCACHE_DISCONNECTED non-directory IS_ROOT dentries we now have
d_unhashed() true.  That makes d_find_alias() logics around
discon_alias dead code:
                if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) {
                        if (IS_ROOT(alias) &&
                            (alias->d_flags & DCACHE_DISCONNECTED)) {
                                discon_alias = alias;
                        } else {
                                __dget_dlock(alias);
                                spin_unlock(&alias->d_lock);
                                return alias;
                        }
                }

Directory case is a red herring - we'll end up picking one and only
alias, whether it's IS_ROOT/disconnected or not.  For non-directories,
though, IS_ROOT and disconnected implies d_unhashed() now, so we might
as well turn that into
	if directory
		return d_find_any_alias
	else
		return the first hashed alias

Does any of the d_find_alias() callers want those dentries?  That is,
non-directories from d_obtain_alias() still not attached to a parent;
note that exportfs_decode_fh() is *not* one of such places - we don't
use d_find_alias() there at all.  If there's no such caller, we can
bloody well just drop the discon_alias logics and be done with that;
if there is, that commit has introduced a bug.  I might have missed
a part of threads related to that patch, so my apologies if it had
been discussed.

Neil, what's the situation there?  A lot of those callers clearly treat
the "only disconnected IS_ROOT alias exist" same as "no aliases at all";
it looks like the change might have been the right thing, but it sure
as hell shouldn't be an undocumented side effect...

  reply	other threads:[~2018-03-12 20:05 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
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 [this message]
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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