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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Aug 16, 2010 at 11:07:37AM -0400, Mathieu Desnoyers wrote:
> * 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))

First, thank you for looking this over!!!

> 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.

You are correct -- I got too aggressive in eliminating synchronization.

Good catch!!!

I added an ACCESS_ONCE() to the second term of the "if" condition so
that it now reads:

	if (--t->rcu_read_lock_nesting == 0 &&
	    unlikely((ACCESS_ONCE(t->rcu_read_unlock_special)))

This prevents the compiler from reordering because the ACCESS_ONCE()
prohibits accessing t->rcu_read_unlock_special unless the value of
t->rcu_read_lock_nesting is known to be zero.

>                                       We might need to audit
> TREE PREEMPT RCU for the same kind of behavior.

The version of __rcu_read_unlock() in kernel/rcutree_plugin.h is as
follows:

void __rcu_read_unlock(void)
{
	struct task_struct *t = current;

	barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
	if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
	    unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
		rcu_read_unlock_special(t);
#ifdef CONFIG_PROVE_LOCKING
	WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
#endif /* #ifdef CONFIG_PROVE_LOCKING */
}

The ACCESS_ONCE() calls should cover this.  I believe that the first
ACCESS_ONCE() is redundant, and have checking this more closely on my
todo list.

> 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.

But rcu_read_unlock_special() does not block.  In fact, it disables
interrupts over almost all of its execution.  Or am I missing some
subtlety here?

> Maybe we should consider clearing rcu_read_unlock_special here ?

If the task blocked in an RCU read-side critical section just before
exit_rcu() was called, we need to remove the task from the ->blkd_tasks
list.  If we fail to do so, we might get a segfault later on.  Also,
we do need to handle any RCU_READ_UNLOCK_NEED_QS requests from the RCU
core.

So I really do like the current approach of calling rcu_read_unlock()
to do this sort of cleanup.

							Thanx, Paul

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