netdev - Re: [PATCH net] af_packet: use spin_lock_bh() for sk_buff

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Date:	Wed, 27 Nov 2013 08:17:55 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Veaceslav Falico <vfalico@...hat.com>
Cc:	netdev@...r.kernel.org, jstancek@...hat.com,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Phil Sutter <phil@....cc>, Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] af_packet: use spin_lock_bh() for sk_buff_head

On Wed, 2013-11-27 at 13:57 +0100, Veaceslav Falico wrote:
> While running a debug kernel, a warning of inconsistent lock state showed
> up for sk->sk_receive_queue->lock between packet_set_ring() (actually,
> static prb_shutdown_retire_blk_timer(), which does spin_lock()) and
> sock_queue_rcv_skb(), which does spin_lock_irqsave().
> 
> Fix this by converting the spin_lock() used in
> prb_shutdown_retire_blk_timer() to spin_lock_bh(), as it is used in
> packet_set_ring().

Patch is good, but description is slightly misleading.

Problem is not because sock_queue_rcv_skb() uses spin_lock_irqsave().

Problem is prb_retire_rx_blk_timer_expired() is called from softirq
context.

So if we do not block BH in prb_shutdown_retire_blk_timer() :

spin_lock(&rb_queue->lock); 
pkc->delete_blk_timer = 1;
spin_unlock(&rb_queue->lock); 

Timer could fire right in the middle and would spin forever trying
to acquire the same lock.

Using spin_lock_bh() prevents timer being fired on this cpu.

Thanks !


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