linux-kernel - Re: deadlock in synchronize

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Message-Id: <20170323153758.GE3637@linux.vnet.ibm.com>
Date:   Thu, 23 Mar 2017 08:37:58 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Nicolai Stange <nicstange@...il.com>,
        gregkh <gregkh@...uxfoundation.org>
Subject: Re: deadlock in synchronize_srcu() in debugfs?

On Thu, Mar 23, 2017 at 03:54:46PM +0100, Johannes Berg wrote:
> Hi,
> 
> Before I go hunting - has anyone seen a deadlock in synchronize_srcu()
> in debugfs_remove() before? We're observing that with our (backported,
> but very recent) driver against 4.9 (and 4.10, I think), but there are
> no backports of any debugfs things so the backport itself doesn't seem
> like a likely problem.
> 
> sysrq-w shows a lot of tasks blocked on various locks (e.g. RTNL), but
> the ultimate problem is the wireless stack getting blocked on
> debugfs_remove_recursive(), in __synchronize_srcu(), in
> wait_for_completion() (while holding lots of locks, hence the other
> tasks getting stuck).

I have not seen this, but my usual question for __synchronize_srcu()
is if some other task is blocked holding srcu_read_lock() for that
same srcu_struct.

							Thanx, Paul