linux-kernel - Re: deadlock in synchronize

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Message-ID: <1490282991.2766.7.camel@sipsolutions.net>
Date:   Thu, 23 Mar 2017 16:29:51 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     linux-kernel <linux-kernel@...r.kernel.org>
Cc:     Nicolai Stange <nicstange@...il.com>,
        "Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
        gregkh <gregkh@...uxfoundation.org>, sharon.dvir@...el.com
Subject: Re: deadlock in synchronize_srcu() in debugfs?

On Thu, 2017-03-23 at 15:54 +0100, Johannes Berg wrote:

> Before I go hunting - has anyone seen a deadlock in
> synchronize_srcu() in debugfs_remove() before?

Isn't it possible for the following to happen?

CPU1					CPU2

mutex_lock(&M);
					full_proxy_xyz();
					srcu_read_lock(&debugfs_srcu);
					real_fops->xyz();
					mutex_lock(&M);
debugfs_remove(F);
synchronize_srcu(&debugfs_srcu);

-> deadlock?

I'm not convinced that this is the scenario I'm looking at, since then
it seems I should see the mutex_lock(&M) on CPU 2 with a backtrace
pointing to a full_proxy and the debugfs operation I recognize, but
lots of debugfs files acquire locks and it seems likely that it's not
always removed without holding those locks?

Am I missing something? I'll see if I can add lockdep annotations.

johannes