netdev - Re: [PATCH] Fix deadlock between boomerang_interrupt and boomerang_start

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Message-ID: <20100823202452.GC12906@hmsreliant.think-freely.org>
Date:	Mon, 23 Aug 2010 16:24:52 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] Fix deadlock between boomerang_interrupt and
 boomerang_start_tx in 3c59x

On Mon, Aug 23, 2010 at 10:01:34PM +0200, Eric Dumazet wrote:
> Le mercredi 11 août 2010 à 11:12 -0400, Neil Horman a écrit :
> > If netconsole is in use, there is a possibility for deadlock in 3c59x between
> > boomerang_interrupt and boomerang_start_xmit.  Both routines take the vp->lock,
> > and if netconsole is in use, a pr_* call from the boomerang_interrupt routine
> > will result in the netconsole code attempting to trnasmit an skb, which can try
> > to take the same spin lock, resulting in deadlock.
> > 
> > The fix is pretty straightforward.  This patch allocats a bit in the 3c59x
> > private structure to indicate that its handling an interrupt.  If we get into
> > the transmit routine and that bit is set, we can be sure that we have recursed
> > and will deadlock if we continue, so instead we just return NETDEV_TX_BUSY, so
> > the stack requeues the skb to try again later.
> > 
> > Signed-off-by: Neil Horman <nhorman@...driver.com>
> > ---
> >  drivers/net/3c59x.c |   15 ++++++++++++++-
> >  1 files changed, 14 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/net/3c59x.c b/drivers/net/3c59x.c
> > index c754d88..c685a55 100644
> > --- a/drivers/net/3c59x.c
> > +++ b/drivers/net/3c59x.c
> > @@ -633,7 +633,8 @@ struct vortex_private {
> >  		open:1,
> >  		medialock:1,
> >  		must_free_region:1,				/* Flag: if zero, Cardbus owns the I/O region */
> > -		large_frames:1;			/* accept large frames */
> > +		large_frames:1,			/* accept large frames */
> > +		handling_irq:1;			/* private in_irq indicator */
> 
> 
> It would be safer and faster to use a dedicated 'int' instead of a
> bitfield.
> 
> 
> 
> 


Faster I agree with, although I'm not sure if speed is really a big issue here,
given that this is a ancient (but fairly well used) 10/100 adapter.  And we
still have space in the octet that that bitfield is living in, so I figured I'd
use that anyway.

As for safe, I'm not sure I follow you on that point.  Is there something
inherently dangerous about using a bitfield in this case?

Neil

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