Re: [PATCH] a local-timer-free version of RCU

[Frederic Weisbecker]

On Sun, Nov 07, 2010 at 06:54:00PM -0800, Paul E. McKenney wrote:
> On Mon, Nov 08, 2010 at 03:19:36AM +0100, Udo A. Steinberg wrote:
> > On Mon, 8 Nov 2010 03:11:36 +0100 Udo A. Steinberg (UAS) wrote:
> > 
> > UAS> On Sat, 6 Nov 2010 12:28:12 -0700 Paul E. McKenney (PEM) wrote:
> > UAS> 
> > UAS> PEM> > + * rcu_quiescent() is called from rcu_read_unlock() when a
> > UAS> PEM> > + * RCU batch was started while the rcu_read_lock/rcu_read_unlock
> > UAS> PEM> > + * critical section was executing.
> > UAS> PEM> > + */
> > UAS> PEM> > +
> > UAS> PEM> > +void rcu_quiescent(int cpu)
> > UAS> PEM> > +{
> > UAS> PEM> 
> > UAS> PEM> What prevents two different CPUs from calling this concurrently?
> > UAS> PEM> Ah, apparently nothing -- the idea being that
> > UAS> PEM> rcu_grace_period_complete() sorts it out.  Though if the second
> > UAS> PEM> CPU was delayed, it seems like it might incorrectly end a
> > UAS> PEM> subsequent grace period as follows:
> > UAS> PEM> 
> > UAS> PEM> o	CPU 0 clears the second-to-last bit.
> > UAS> PEM> 
> > UAS> PEM> o	CPU 1 clears the last bit.
> > UAS> PEM> 
> > UAS> PEM> o	CPU 1 sees that the mask is empty, so invokes
> > UAS> PEM> 	rcu_grace_period_complete(), but is delayed in the function
> > UAS> PEM> 	preamble.
> > UAS> PEM> 
> > UAS> PEM> o	CPU 0 sees that the mask is empty, so invokes
> > UAS> PEM> 	rcu_grace_period_complete(), ending the grace period.
> > UAS> PEM> 	Because the RCU_NEXT_PENDING is set, it also starts
> > UAS> PEM> 	a new grace period.
> > UAS> PEM> 
> > UAS> PEM> o	CPU 1 continues in rcu_grace_period_complete(),
> > UAS> PEM> incorrectly ending the new grace period.
> > UAS> PEM> 
> > UAS> PEM> Or am I missing something here?
> > UAS> 
> > UAS> The scenario you describe seems possible. However, it should be easily
> > UAS> fixed by passing the perceived batch number as another parameter to
> > UAS> rcu_set_state() and making it part of the cmpxchg. So if the caller
> > UAS> tries to set state bits on a stale batch number (e.g., batch !=
> > UAS> rcu_batch), it can be detected.
> > UAS> 
> > UAS> There is a similar, although harmless, issue in call_rcu(): Two CPUs can
> > UAS> concurrently add callbacks to their respective nxt list and compute the
> > UAS> same value for nxtbatch. One CPU succeeds in setting the PENDING bit
> > UAS> while observing COMPLETE to be clear, so it starts a new batch.
> > 
> > Correction: while observing COMPLETE to be set!
> > 
> > UAS> Afterwards, the other CPU also sets the PENDING bit, but this time for
> > UAS> the next batch. So it ends up requesting nxtbatch+1, although there is
> > UAS> no need to. This also would be fixed by making the batch number part of
> > UAS> the cmpxchg.
> 
> Another approach is to map the underlying algorithm onto the TREE_RCU
> data structures.  And make preempt_disable(), local_irq_save(), and
> friends invoke rcu_read_lock() -- irq and nmi handlers already have
> the dyntick calls into RCU, so should be easy to handle as well.
> Famous last words.  ;-)


So, this looks very scary for performances to add rcu_read_lock() in
preempt_disable() and local_irq_save(), that notwithstanding it won't
handle the "raw" rcu sched implicit path. We should check all rcu_dereference_sched
users to ensure there are not in such raw path.


There is also my idea from the other discussion: change rcu_read_lock_sched()
semantics and map it to rcu_read_lock() in this rcu config (would be a nop
in other configs). So every users of rcu_dereference_sched() would now need
to protect their critical section with this.
Would it be too late to change this semantic?

What is scary with this is that it also changes rcu sched semantics, and users
of call_rcu_sched() and synchronize_sched(), who rely on that to do more
tricky things than just waiting for rcu_derefence_sched() pointer grace periods,
like really wanting for preempt_disable and local_irq_save/disable, those
users will be screwed... :-(  ...unless we also add relevant rcu_read_lock_sched()
for them...

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