linux-kernel - Re: [git pull] vfs.git spurious ENOTDIR fix

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Message-ID: <20150802043947.GB17109@ZenIV.linux.org.uk>
Date:	Sun, 2 Aug 2015 05:39:47 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dominique Martinet <asmadeus@...ewreck.org>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Dominique Martinet <dominique.martinet@....fr>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [git pull] vfs.git spurious ENOTDIR fix

On Sat, Aug 01, 2015 at 09:06:55PM -0700, Hugh Dickins wrote:

> (I don't actually understand why the clearing of DCACHE_ENTRY_TYPE in
> dentry_iput() is not of continuing concern; but don't worry, there's
> plenty I don't understand - so long as you're both satisfied that
> it's not a concern, no need to persuade me.)

Because before we even get to dentry_iput(), we evict the fucker from hash.
And that will do dentry_rcuwalk_invalidate(dentry), which will bump ->d_seq
*AFTER* having it unhashed.  Now look at __d_lookup_rcu() - there we fetch
->d_seq, then verify that it's still hashed.

So having hit dentry_iput() means that everyone who'd found it via RCU
lookup will be guaranteed a ->d_seq mismatch.  The same goes for things
like d_drop() and d_instantiate().  Look for dentry_rcuwalk_invalidate()
callers in there...

Clearing DCACHE_ENTRY_TYPE there is fine - dentry *is* made negative there,
after all.  What we want is to have ->d_inode stable at least as long as
->d_seq remains so.
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