Re: [PATCH] rcu: exp: Fix "must hold exp_mutex" comments for QS reporting functions

[Boqun Feng]

On Thu, Mar 08, 2018 at 07:42:55AM -0800, Paul E. McKenney wrote:
> On Thu, Mar 08, 2018 at 04:30:06PM +0800, Boqun Feng wrote:
> > On Thu, Mar 08, 2018 at 12:54:29PM +0800, Boqun Feng wrote:
> > > On Wed, Mar 07, 2018 at 08:30:17PM -0800, Paul E. McKenney wrote:
> > > [...]
> > > > >  
> > > > > +/*
> > > > > + * Like sync_rcu_preempt_exp_done(), but this function assumes the caller
> > > > > + * doesn't hold the rcu_node's ->lock, and will acquire and release the lock
> > > > > + * itself
> > > > > + */
> > > > > +static bool sync_rcu_preempt_exp_done_unlocked(struct rcu_node *rnp)
> > > > > +{
> > > > > +	unsigned long flags;
> > > > > +	bool ret;
> > > > > +
> > > > > +	raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > > > > +	ret = sync_rcu_preempt_exp_done(rnp);
> > > > 
> > > > Let's see...  The sync_rcu_preempt_exp_done() function checks the
> > > > ->exp_tasks pointer and the ->expmask bitmask.  The number of bits in the
> > > > mask can only decrease, and the ->exp_tasks pointer can only transition
> > > > from NULL to non-NULL when there is at least one bit set.  However,
> > > > there is no ordering in sync_rcu_preempt_exp_done(), so it is possible
> > > > that it could be fooled without the lock:
> > > > 
> > > > o	CPU 0 in sync_rcu_preempt_exp_done() reads ->exp_tasks and
> > > > 	sees that it is NULL.
> > > > 
> > > > o	CPU 1 blocks within an RCU read-side critical section, so
> > > > 	it enqueues the task and points ->exp_tasks at it and
> > > > 	clears CPU 1's bit in ->expmask.
> > > > 
> > > > o	All other CPUs clear their bits in ->expmask.
> > > > 
> > > > o	CPU 0 reads ->expmask, sees that it is zero, so incorrectly
> > > > 	concludes that all quiescent states have completed, despite
> > > > 	the fact that ->exp_tasks is non-NULL.
> > > > 
> > > > So it seems to me that the lock is needed.  Good catch!!!  The problem
> > > > would occur only if the task running on CPU 0 received a spurious
> > > > wakeup, but that could potentially happen.
> > > 
> > > Thanks for the analysis ;-)
> 
> The other limitation is that it occurs only on systems small enough
> to have a single-node rcu_node tree.  But still...
> 
> > > > If lock contention becomes a problem, memory-ordering tricks could be
> > > > applied, but the lock is of course simpler.
> > > > 
> > > 
> > > Agreed.
> > > 
> > > > I am guessing that this is a prototype patch, and that you are planning
> > > 
> > > Yes, this is a prototype. And I'm preparing a proper patch to send
> > > later.
> 
> Very good, thank you!
> 
> > > > to add lockdep annotations in more places, but either way please let
> > > > me know.
> > > 
> > > Give it's a bug as per your analysis, I'd like to defer other lockdep
> > > annotations and send this first. However, I'm currently getting other
> > > lockdep splats after applying this, so I need to get that sorted first.
> > 
> > Hmm.. the other lockdep splat seems irrelevant with my patch, I could
> > observe it on mainline using rcutorture with CONFIG_PROVE_LOCKING=y. I'd
> > spend some more time on it, in the meanwhile, send a proper patch for
> > this sync_rcu_preempt_exp_done().
> 
> I am not seeing that one, but am very interested in getting it fixed!  ;-)
> 

Found the root cause, and send out the patch ;-)

Regards,
Boqun

> 							Thanx, Paul
>

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