Re: [Lse-tech] [RFC, PATCH] 2/5 rcu lock update: Use a sequence lock for starting batches

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

On Fri, May 28, 2004 at 03:57:07PM +0200, Manfred Spraul wrote:
> Paul E. McKenney wrote:
> 
> >Hello, Manfred,
> >
> >I am still digging through these, and things look quite good in general,
> >but I have a question on your second patch.

Sorry for the bother, will keep looking, hopefully with more accurate
comments in the future.  :-/

						Thanx, Paul

> Let's assume that
> 
> batch.completed = 5;
> batch.cur = 5;
> batch.next_pending = 0;
> 
> >Given the following sequence of events:
> >
> >1.	CPU 0 executes the
> >
> >		rcu_ctrlblk.batch.next_pending = 1;
> > 
> >
> batch.next_pending = 1.
> 
> >	at the beginning of rcu_start_batch().
> >
> >2.	CPU 1 executes the read_seqcount code sequence in
> >	rcu_process_callbacks(), setting RCU_batch(cpu) to
> >	the next batch number, and setting next_pending to 1.
> > 
> >
> RCU_batch(1) is now 6.
> next_pending is 1, rcu_process_callbacks continues without calling 
> rcu_start_batch().
> 
> >3.	CPU 0 executes the remainder of rcu_start_batch(),
> >	setting rcu_ctrlblk.batch.next_pending to 0 and
> >	incrementing rcu_ctrlblk.batch.cur.
> > 
> >
> batch.cur = 6.
> 
> >4.	CPU 1's state is now as if the grace period had already
> >	completed for the callbacks that were just moved to
> >	RCU_curlist(), which would be very bad.
> > 
> >
> AFAICS: No. RCU_batch(1) is 6 and rcu_ctrlblk.batch.completed is still 
> 5. The test for grace period completed is
> 
> > if (!list_empty(&RCU_curlist(cpu)) &&
> >            
> >!rcu_batch_before(rcu_ctrlblk.batch.completed,RCU_batch(cpu))) {
> >         __list_splice(&RCU_curlist(cpu), &list);
> >         INIT_LIST_HEAD(&RCU_curlist(cpu));
> > }
> 
> 5 is before 6, thus the callbacks won't be processed.
> 
> The only write operation to rcu_ctrlblk.batch.completed is in cpu_quiet, 
> after checking that the cpu bitmap is empty and under 
> spin_lock(rcu_ctrlblk.state.mutex).
> 
> Thanks for looking at my patches,
> 
> --
>    Manfred
> 
>