netdev - Re: [PATCH net-2.6] 3c59x: Remove incorrect locking; correct documented lock hierarchy

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Date:	Wed, 01 Sep 2010 23:08:27 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-2.6] 3c59x: Remove incorrect locking; correct
 documented lock hierarchy

On Wed, 2010-09-01 at 22:47 +0100, Ben Hutchings wrote:
> On Wed, 2010-09-01 at 14:38 -0700, David Miller wrote:
> > From: Ben Hutchings <ben@...adent.org.uk>
> > Date: Tue, 31 Aug 2010 01:15:33 +0100
> > 
> > > vortex_ioctl() was grabbing vortex_private::lock around its call to
> > > generic_mii_ioctl().  This is no longer necessary since there are more
> > > specific locks which the mdio_{read,write}() functions will obtain.
> > > Worse, those functions do not save and restore IRQ flags when locking
> > > the MII state, so interrupts will be enabled when generic_mii_ioctl()
> > > returns.
> > > 
> > > Since there is currently no need for any function to call
> > > mdio_{read,write}() while holding another spinlock, do not change them
> > > to save and restore IRQ flags but remove the specification of ordering
> > > between vortex_private::lock and vortex_private::mii_lock.
> > > 
> > > Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
> > > ---
> > > I've now borrowed a card to test 3c59x on.  I've seen another regression
> > > reported <http://bugs.debian.org/586967> after my locking changes, which
> > > I can't reproduce.
> > 
> > I think the lock is necessary, in some form.
> > 
> > Nothing otherwise protects vp->mii, which is accessed and modified by
> > not just this ioctl, but also ethtool operation calls.
> > 
> > So we can't apply your patch as-is.
> 
> Hmm, yes, I forgot that mii caches information in struct mii_if_info.
> Let me rethink this.

I think this is safe after all because ethtool and MII ioctls are all
serialised by the RTNL.

Ben.

-- 
Ben Hutchings
Once a job is fouled up, anything done to improve it makes it worse.

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