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

[houston . jim]

Hi Everyone,

I'm sorry started this thread and have not been able to keep up
with the discussion.  I agree that the problems described are real.

> > 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.

My thought on how to fix this case is to only hand off the DO_RCU_COMPLETION
to a single cpu.  The rcu_unlock which receives this hand off would clear its
own bit and then call rcu_poll_other_cpus to complete the process.

> 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...

I need to stare at the code and get back up to speed. I expect that the synchronize_sched
path in my patch is just plain broken.

Jim Houston
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