linux-kernel - Re: deadlock in synchronize

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Message-ID: <87o9ws6m4s.fsf@gmail.com>
Date:   Thu, 23 Mar 2017 16:36:19 +0100
From:   Nicolai Stange <nicstange@...il.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Nicolai Stange <nicstange@...il.com>,
        "Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
        gregkh <gregkh@...uxfoundation.org>
Subject: Re: deadlock in synchronize_srcu() in debugfs?

Hi Johannes,

On Thu, Mar 23 2017, Johannes Berg wrote:

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

Not yet. How reproducible is this?

> We're observing that with our (backported, but very recent) driver
> against 4.9 (and 4.10, I think),

Do I understand it correctly that this driver has been backported from
4.11-rcX to 4.9/10 and that there isn't any issue with 4.11-rcX?

> but there are no backports of any debugfs things so the backport
> itself doesn't seem like a likely problem.

Right, there haven't been any SRCU related changes to debugfs after
4.8.

> 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).

Could you share a complete backtrace? For example, is the
debugfs_remove_recursive() called from any debugfs file's fops and thus,
possibly from within a SRCU read side critical section?

Thanks,

Nicolai