linux-kernel - Re: 2.6.39-rc6-mmotm0506 - lockdep splat in RCU code on page fault

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Date:	Wed, 11 May 2011 00:44:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Valdis.Kletnieks@...edu, Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: 2.6.39-rc6-mmotm0506 - lockdep splat in RCU code on page fault

On Tue, May 10, 2011 at 10:44:43PM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Tue, May 10, 2011 at 10:57:46AM +0200, Ingo Molnar wrote:
> > > 
> > > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > > 
> > > > -	raw_spin_lock_irqsave(&rnp->lock, flags);
> > > > -	rnp->wakemask |= rdp->grpmask;
> > > > -	raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > > > +	do {
> > > > +		old = rnp->wakemask;
> > > > +		new = old | rdp->grpmask;
> > > > +	} while (cmpxchg(&rnp->wakemask, old, new) != old);
> > > 
> > > Hm, isnt this an inferior version of atomic_or_long() in essence?
> > > 
> > > Note that atomic_or_long() is x86 only, so a generic one would have to be 
> > > offered too i suspect, atomic_cmpxchg() driven or so - which would look like 
> > > the above loop.
> > > 
> > > Most architectures could offer atomic_or_long() i suspect.
> > 
> > Is the following what you had in mind?  This (untested) patch provides only 
> > the generic function: if this is what you had in mind, I can put together 
> > optimized versions for a couple of the architectures.
> 
> Yeah, something like this, except:
> 
> > +#ifndef CONFIG_ARCH_HAS_ATOMIC_OR_LONG
> > +static inline void atomic_or_long(unsigned long *v1, unsigned long v2)
> > +{
> > +	unsigned long old;
> > +	unsigned long new;
> > +
> > +	do {
> > +		old = ACCESS_ONCE(*v1);
> > +		new = old | v2;
> > +	} while (cmpxchg(v1, old, new) != old);
> > +}
> > +#endif /* #ifndef CONFIG_ARCH_HAS_ATOMIC_OR_LONG */
> 
> Shouldnt that method work on atomic_t (or atomic64_t)?

Works for me -- and in this case it is quite easy to change existing uses.

							Thanx, Paul
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