Re: [PATCH tip/core/rcu 08/10] rcu: Add a TINY_PREEMPT_RCU

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
[...]
> +
> +/*
> + * Tiny-preemptible RCU implementation for rcu_read_unlock().
> + * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
> + * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
> + * invoke rcu_read_unlock_special() to clean up after a context switch
> + * in an RCU read-side critical section and other special cases.
> + */
> +void __rcu_read_unlock(void)
> +{
> +	struct task_struct *t = current;
> +
> +	barrier();  /* needed if we ever invoke rcu_read_unlock in rcutiny.c */
> +	if (--t->rcu_read_lock_nesting == 0 &&
> +	    unlikely(t->rcu_read_unlock_special))

Hrm I think we discussed this in a past life, but would the following
sequence be possible and correct ?

CPU 0

read t->rcu_read_unlock_special
  interrupt comes in, preempts. sets t->rcu_read_unlock_special
  <preempted>
  <scheduled back>
  iret
decrement and read t->rcu_read_lock_nesting
test both old "special" value (which we have locally on the stack) and
detect that rcu_read_lock_nesting is 0.

We actually missed a reschedule.

I think we might need a barrier() between the t->rcu_read_lock_nesting
and t->rcu_read_unlock_special reads. We might need to audit
TREE PREEMPT RCU for the same kind of behavior.

But I might be (again ?) missing something. I've got the feeling you
already convinced me that this was OK for some reason, but I trip on
this every time I read the code.

[...]

> +/*
> + * Check for a task exiting while in a preemptible -RCU read-side
> + * critical section, clean up if so.  No need to issue warnings,
> + * as debug_check_no_locks_held() already does this if lockdep
> + * is enabled.
> + */
> +void exit_rcu(void)
> +{
> +	struct task_struct *t = current;
> +
> +	if (t->rcu_read_lock_nesting == 0)
> +		return;
> +	t->rcu_read_lock_nesting = 1;
> +	rcu_read_unlock();
> +}
> +

The interaction with preemption is unclear here. exit.c disables
preemption around the call to exit_rcu(), but if, for some reason,
rcu_read_unlock_special was set earlier by preemption, then the
rcu_read_unlock() code might block and cause problems.

Maybe we should consider clearing rcu_read_unlock_special here ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com