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.
next prev parent 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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