linux-kernel - Re: sctp: hang in sctp_remaddr_seq

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Message-ID: <20130318203202.GB9478@hmsreliant.think-freely.org>
Date:	Mon, 18 Mar 2013 16:32:02 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>, sri@...ibm.com,
	"David S. Miller" <davem@...emloft.net>,
	linux-sctp@...r.kernel.org, netdev@...r.kernel.org,
	Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: sctp: hang in sctp_remaddr_seq_show

On Mon, Mar 18, 2013 at 11:31:06AM -0400, Vlad Yasevich wrote:
> On 03/18/2013 11:25 AM, Eric Dumazet wrote:
> >On Mon, 2013-03-18 at 07:04 -0400, Neil Horman wrote:
> >
> >>I'm not sure why the process would never get back to the schedule, but looking
> >>at the sctp_remaddr_seq_show function, I think that we should convert this
> >>sequence:
> >>	sctp_local_bh_disable();
> >>	read_lock(&head->lock);
> >>	rcu_read_lock();
> >>
> >>to this:
> >>	read_lock(&head->lock);
> >>	rcu_read_lock_bh();
> >>
> >>Neil
> >
> >I dont think so.
> >
> >BH needs to be disabled before read_lock(&head->lock);
> >
> >or else, write_lock() could deadlock (assuming it can be called from BH)
> >
> >
> 
> If anything, this should probably be done like this:
> 
> rcu_read_lock();
> read_lock_bh(&head->lock)
> ...
> 
> read_unlock_bh(&head->lock)
> rcu_read_unlock();
> 
Vlads, right.  We need to grab the rcu lock before the read lock, but we should
probably use the rcu_read_lock_bh variant, since we're going to disable bottom
halves anyway.
Neil

> -vlad
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