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From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH][RFC] %pd - for printing dentry name
Date: Tue, 2 Feb 2010 01:06:53 +0000	[thread overview]
Message-ID: <20100202010653.GD12882@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20100201231847.GC12882@ZenIV.linux.org.uk>

On Mon, Feb 01, 2010 at 11:18:47PM +0000, Al Viro wrote:

> Ehh...  RCU will save you from stepping on freed memory, but it still will
> leave the joy of half-updated string with length out of sync with it, etc.
> We probably can get away with that, but we'll have to be a lot more careful
> with the order of updating these suckers in d_move_locked et.al.
> 
> I don't know...  Note that if we end up adding something extra to struct
> dentry, we might as well just add *another* spinlock, taken only under
> ->d_lock and only in two places in dcache.c that change d_name.  That kind
> of thing is trivial to enforce (just grep over the tree once in a while)
> and if it shares the cacheline with d_lock, we shouldn't get any real overhead
> in d_move()/d_materialise_unique().  I'm not particulary fond of that variant,
> but it's at least guaranteed to be devoid of subtleties.
> 
> If RCU folks can come up with a sane suggestions that would be robust and
> wouldn't bloat dentry - sure, I'm all for it.  If not...

As the matter of fact, there's just *one* place that has any business [*]
changing ->d_name contents of dentry that might be visible to somebody
else.  fs/dcache.c::switch_names().

So a very brute-force approach would be to add a new spinlock to dentry and
have switch_names() grab it on dentry and target and drop when we are done,
dname_string() grab it around the call of string() and pull the guts out
through the nose to anyone who as much as mentions that lock outside of
fs/dcache.c:switch_names() and lib/vsprintf.c:dname_string().

Again, I'd love to see something more elegant; this variant won't add any
contention and if we place the lock next to d_lock we won't get any cacheline
bouncing either (we'd just taken ->d_lock on both dentries), but it's
rather ugly way to deal with the problem.  I mean, a spinlock just for the
needs of debugging printks?  Yuck.

BTW, speaking of ->d_lock, dget_parent() is abused in a bunch of places.
I'm going through review of ->d_parent and ->d_name uses; will post
the results when it's done...

[*] there's also !@$#!@#!@# {ncp,smb}_fill_cache() that does change of
letters' case in ->d_name; no locking whatsoever in there, luckily for
that crap the callers hold i_mutex on parent, so they get exclusion with
potential callers of d_move().  Bad Idea All Around(tm), but irrelevant
for our purposes.

  reply	other threads:[~2010-02-02  1:06 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-02-01 22:25 [PATCH][RFC] %pd - for printing dentry name Al Viro
2010-02-01 22:34 ` Al Viro
2010-02-01 22:37 ` Linus Torvalds
2010-02-01 23:18   ` Al Viro
2010-02-02  1:06     ` Al Viro [this message]
2010-02-02  5:55       ` Eric W. Biederman
2010-02-02 17:01         ` Al Viro
2010-02-02 18:10           ` Olivier Galibert
2010-02-02 19:19           ` Eric W. Biederman
2010-02-03  3:04             ` Al Viro
2010-02-04  4:53               ` Al Viro
2010-02-02  4:22     ` Linus Torvalds
2010-02-02  5:00       ` Al Viro
2010-02-02  6:36         ` Nick Piggin
2010-02-04  6:02           ` Al Viro
2010-02-04  7:40             ` Nick Piggin
2010-02-02  6:53     ` Paul E. McKenney
2010-02-02  7:09       ` Al Viro
2010-02-02 13:32         ` Matthew Wilcox
2010-02-02 15:56           ` Linus Torvalds
2010-02-02 16:13             ` Matthew Wilcox
2010-02-02 16:43           ` Al Viro
2010-02-03 10:52         ` Paul E. McKenney
2010-02-03  2:49       ` Paul E. McKenney
2010-02-04 15:29         ` Linus Torvalds
2010-02-04 16:02           ` Paul E. McKenney
2010-02-04 17:13             ` Linus Torvalds
2010-02-04 17:36               ` Al Viro
2010-02-07 16:34                 ` Paul E. McKenney
2010-02-01 22:45 ` Joe Perches

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