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

[Paul E. McKenney]

On Mon, Aug 16, 2010 at 06:07:05PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> > On Mon, Aug 16, 2010 at 05:41:23PM -0400, Mathieu Desnoyers wrote:
> > > * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> > > > On Mon, Aug 16, 2010 at 03:19:47PM -0400, Mathieu Desnoyers wrote:
> > > > > * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> > > > > > On Mon, Aug 16, 2010 at 11:07:37AM -0400, Mathieu Desnoyers wrote:
> > > > > > > * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) 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.
> > > > > 
> > > > > Hrm, --t->rcu_read_lock_nesting does not have any globally visible
> > > > > side-effect, so the compiler is free to reorder the memory access across
> > > > > the rcu_read_unlock_special access. I think we need the ACCESS_ONCE()
> > > > > around the t->rcu_read_lock_nesting access too.
> > > > 
> > > > Indeed, it is free to reorder that access.  This has the effect of
> > > > extending the scope of the RCU read-side critical section, which is
> > > > harmless as long as it doesn't pull a lock or some such into it.
> > > > 
> > > 
> > > So what happens if we get:
> > > 
> > > CPU 0
> > > 
> > > read t->rcu_read_lock_nesting
> > > check if equals to 1
> > > read t->rcu_read_unlock_special
> > >   interrupt comes in, preempts. sets t->rcu_read_unlock_special
> > >   <preempted>
> > >   <scheduled back>
> > >   iret
> > > decrement t->rcu_read_lock_nesting
> > 
> > Moving this down past the check of t->rcu_read_lock_special (which is
> > now covered by ACCESS_ONCE()) would violate the C standard, as it would
> > be equivalent to moving a volatile up past a sequence point.
> 
> Hrm, I'm not quite convinced yet. I am not concerned about gcc moving
> the volatile access prior to the sequence point (as you say, this is
> forbidden by the C standard), but rather that:
> 
> --(t->rcu_read_lock_nesting)
> 
> could be split in two distinct operations:
> 
> read t->rcu_read_lock_nesting
> decrement t->rcu_read_lock_nesting
> 
> Note that in order to know the result required to pass the sequence
> point "&&" (the test), we only need to perform the read, not the
> decrement. AFAIU, gcc would be in its rights to move the
> t->rcu_read_lock_nesting update after the volatile access.

I will run this by some compiler experts.

							Thanx, Paul
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