linux-kernel - Q. locking order of dcache_lru

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Message-ID: <9769.1302268831@jrobl>
Date:	Fri, 08 Apr 2011 22:20:31 +0900
From:	"J. R. Okajima" <hooanon05@...oo.co.jp>
To:	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Nick Piggin <npiggin@...e.de>
cc:	linux-kernel@...r.kernel.org
Subject: Q. locking order of dcache_lru_lock


Hello Al Viro, Christoph Hellwig and Nick Piggin,

I have a question about the locking order of dcache_lru_lock.

The comment in fs/dcache.c says
 * Ordering:
 * dentry->d_inode->i_lock
 *   dentry->d_lock
 *     dcache_lru_lock
	:::

d_lock should be before dcache_lru_lock.
Actually dentry_lru_(add|del|move_tail) functions (and their callers) do
it expectedly.
But __shrink_dcache_sb() looks different.

__shrink_dcache_sb()
{
	:::
relock:
	spin_lock(&dcache_lru_lock);
	while (!list_empty(&sb->s_dentry_lru)) {
		::
		if (!spin_trylock(&dentry->d_lock)) {
			spin_unlock(&dcache_lru_lock);
			cpu_relax();
			goto relock;
		}
	:::
}

When spin_trylock(&dentry->d_lock) successfully acquired d_lock, does
the violation of locking order happen (or a deadlock, in worse case)?

By the way, the code is introduced by the commit
	2304450 2011-01-07 fs: dcache scale lru
by Nick Piggin.
Is he allright? Does anyone know anything?
We have not received from him for a long time.


J. R. Okajima
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