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Date:   Fri, 07 Feb 2020 11:22:04 +0100 (CET)
From:   David Miller <davem@...emloft.net>
To:     dhowells@...hat.com
Cc:     netdev@...r.kernel.org, linux-afs@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] rxrpc: Fix call RCU cleanup using non-bh-safe locks

From: David Howells <dhowells@...hat.com>
Date: Thu, 06 Feb 2020 13:57:40 +0000

> rxrpc_rcu_destroy_call(), which is called as an RCU callback to clean up a
> put call, calls rxrpc_put_connection() which, deep in its bowels, takes a
> number of spinlocks in a non-BH-safe way, including rxrpc_conn_id_lock and
> local->client_conns_lock.  RCU callbacks, however, are normally called from
> softirq context, which can cause lockdep to notice the locking
> inconsistency.
> 
> To get lockdep to detect this, it's necessary to have the connection
> cleaned up on the put at the end of the last of its calls, though normally
> the clean up is deferred.  This can be induced, however, by starting a call
> on an AF_RXRPC socket and then closing the socket without reading the
> reply.
> 
> Fix this by having rxrpc_rcu_destroy_call() punt the destruction to a
> workqueue if in softirq-mode and defer the destruction to process context.
> 
> Note that another way to fix this could be to add a bunch of bh-disable
> annotations to the spinlocks concerned - and there might be more than just
> those two - but that means spending more time with BHs disabled.
> 
> Note also that some of these places were covered by bh-disable spinlocks
> belonging to the rxrpc_transport object, but these got removed without the
> _bh annotation being retained on the next lock in.
> 
> Fixes: 999b69f89241 ("rxrpc: Kill the client connection bundle concept")
> Reported-by: syzbot+d82f3ac8d87e7ccbb2c9@...kaller.appspotmail.com
> Reported-by: syzbot+3f1fd6b8cbf8702d134e@...kaller.appspotmail.com
> Signed-off-by: David Howells <dhowells@...hat.com>

Applied and queued up for -stable.

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