linux-kernel - Re: Heads-up: 3.6.2 / 3.6.3 NFS server oops: 3.6.2+ regression? (also an unrelated ext4 data loss bug)

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Message-ID: <87vce1w241.fsf@spindle.srvr.nix>
Date:	Tue, 23 Oct 2012 18:05:18 +0100
From:	Nix <nix@...eri.org.uk>
To:	"Myklebust\, Trond" <Trond.Myklebust@...app.com>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	"Ted Ts'o" <tytso@....edu>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"Schumaker\, Bryan" <Bryan.Schumaker@...app.com>,
	Peng Tao <bergwolf@...il.com>,
	"gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-nfs\@vger.kernel.org" <linux-nfs@...r.kernel.org>,
	Stanislav Kinsbursky <skinsbursky@...allels.com>
Subject: Re: Heads-up: 3.6.2 / 3.6.3 NFS server oops: 3.6.2+ regression? (also an unrelated ext4 data loss bug)

On 23 Oct 2012, Trond Myklebust spake thusly:
> On Tue, 2012-10-23 at 12:46 -0400, J. Bruce Fields wrote:
>> Looks like there's some confusion about whether nsm_client_get() returns
>> NULL or an error?
>
> nsm_client_get() looks extremely racy in the case where ln->nsm_users ==
> 0.  Since we never recheck the value of ln->nsm_users after taking
> nsm_create_mutex, what is stopping 2 different threads from both setting
> ln->nsm_clnt and re-initialising ln->nsm_users?

Yep. At the worst possible time:

	spin_lock(&ln->nsm_clnt_lock);
	if (ln->nsm_users) {
		if (--ln->nsm_users)
			ln->nsm_clnt = NULL;
(1)		shutdown = !ln->nsm_users;
	}
	spin_unlock(&ln->nsm_clnt_lock);

If a thread reinitializes nsm_users at point (1), after the assignment,
we could well end up with ln->nsm_clnt NULL and shutdown false. A bit
later, nsm_mon_unmon gets called with a NULL clnt, and boom.

This seems particularly likely if there is only one nsm_user (which is
true in my case, since I have only one active network namespace).

-- 
NULL && (void)
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