netdev - Re: [irda-users] [BUG] 2.6.20.1-rt8 irnet + pppd recursive spinlock...

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Message-ID: <20070407005926.GE4177@sortiz.org>
Date:	Sat, 7 Apr 2007 03:59:26 +0300
From:	Samuel Ortiz <samuel@...tiz.org>
To:	Guennadi Liakhovetski <gl@...-ac.de>
Cc:	Paul Mackerras <paulus@...ba.org>,
	irda-users@...ts.sourceforge.net, linux-rt-users@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [irda-users] [BUG] 2.6.20.1-rt8 irnet + pppd recursive spinlock...

Hi Guennadi,

On Thu, Apr 05, 2007 at 12:59:40PM +0200, Guennadi Liakhovetski wrote:
> Ok, a simple analysis reveals the recursive spinlock:
> 
> On Thu, 5 Apr 2007, Guennadi Liakhovetski wrote:
> 
> > [<bf12b220>] (ppp_channel_push+0x0/0xc8 [ppp_generic]) from [<bf12bf98>] (ppp_output_wakeup+0x18/0x1c [ppp_generic])
> ===>		^^^^^^^^^^^^^^^^
> >  r7 = C38F42BC  r6 = C38F4200  r5 = C38F4200  r4 = 00000000
> 
> ===>	spin_lock_bh(&pch->downl);
> 
> > [<bf12bf80>] (ppp_output_wakeup+0x0/0x1c [ppp_generic]) from [<bf132c98>] (irnet_flow_indication+0x38/0x3c [irnet])
> > [<bf132c60>] (irnet_flow_indication+0x0/0x3c [irnet]) from [<bf104e4c>] (irttp_run_tx_queue+0x1c0/0x1d4 [irda])
> > [<bf104c8c>] (irttp_run_tx_queue+0x0/0x1d4 [irda]) from [<bf104f88>] (irttp_data_request+0x128/0x4f8 [irda])
> >  r8 = BF121560  r7 = 00000002  r6 = C38F4200  r5 = C21418B8
> >  r4 = C21418B8 
> > [<bf104e60>] (irttp_data_request+0x0/0x4f8 [irda]) from [<bf1321bc>] (ppp_irnet_send+0x134/0x238 [irnet])
> > [<bf132088>] (ppp_irnet_send+0x0/0x238 [irnet]) from [<bf12a600>] (ppp_push+0x80/0xb8 [ppp_generic])
> >  r7 = C3A436E0  r6 = 00000000  r5 = C21418B8  r4 = C1489600
> > [<bf12a580>] (ppp_push+0x0/0xb8 [ppp_generic]) from [<bf12a8d8>] (ppp_xmit_process+0x34/0x50c [ppp_generic])
> ===>		^^^^^^^^
> >  r7 = 00000021  r6 = C21418B8  r5 = C1489600  r4 = 00000000
> 
> ===>		spin_lock_bh(&pch->downl);
> 
> > [<bf12a8a4>] (ppp_xmit_process+0x0/0x50c [ppp_generic]) from [<bf12aed8>] (ppp_start_xmit+0x128/0x254 [ppp_generic])
> > [<bf12adb0>] (ppp_start_xmit+0x0/0x254 [ppp_generic]) from [<c0186fa4>] (dev_hard_start_xmit+0x170/0x268)
> > [<c0186e34>] (dev_hard_start_xmit+0x0/0x268) from [<c01979b8>] (__qdisc_run+0x60/0x270)
> >  r8 = C1BBC914  r7 = C21418B8  r6 = 00000000  r5 = C21418B8
> >  r4 = C1BBC800 
> > [<c0197958>] (__qdisc_run+0x0/0x270) from [<c0187250>] (dev_queue_xmit+0x1b4/0x25c)
> 
> Now, does anyone have an idea how best to fix it?
> 
> 1. Should re-entrance in ppp_channel_push() be prevented, and if yes - at 
> which level? Or
> 
> 2. Should re-entrance be allowed and only recursive spin_lock_bh() 
> avoided?
IMHO, irnet_flow_indication() should be called asynchronously by
irttp_run_tx_queue(), through some bottom-half mechanism. That would fix your
locking issues, and that would reduce the time we spend in the IrDA code with
this lock taken.

I will try to come up with some patches for you later this weekend.

Cheers,
Samuel.

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