Re: VFS deadlock ?

From: Al Viro
Date: Thu Mar 21 2013 - 20:08:11 EST

On Thu, Mar 21, 2013 at 04:58:41PM -0700, Linus Torvalds wrote:

> And the only other reason we don't want to allow it is to make sure
> you can't have directory loops etc, afaik, and again, for this
> particular case of /proc, we happen to be ok.

Not really. Do that and yes, this deadlock goes away. But the locking
order in general goes to hell - we order directory inodes by "which dentry
is an ancestor of another?" So we have no warranty that we won't get
alias1/foo/bar/baz < alias2/foo. Take rename_lock() on those two and
have it race with rmdir alias2/foo/bar/baz (locks alias2/foo/bar, then
alias2/foo/bar/baz) and rmdir alias2/foo/bar (locks alias2/foo and
alias2/foo/bar). Oops - we have a cycle now...
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