Re: [PATCH 1/2] srcu: RCU variant permitting read-side blocking

[Oleg Nesterov <oleg@tv-sign.ru>]

On 06/27, Paul E. McKenney wrote:
>
> On Wed, Jun 28, 2006 at 01:13:58AM +0400, Oleg Nesterov wrote:
> > 
> > Also, I can't understand the purpose of 2-nd synchronize_sched() in
> > synchronize_srcu().
> 
> This one handles the srcu_read_unlock() analog of the situation you
> are worried about above.  The reader does not have memory barriers in
> srcu_read_unlock(), so an access to the data structure might get
> reordered to follow the decrement of .c[0] -- which would get messed
> up by the following kfree().

Aha, I see.

The last question. The 'srcu-2' you posted today does synchronize_srcu_flip()
twice. You did it this way because srcu is optimized for readers, otherwise we
could just add smp_rmb() into srcu_read_lock() - this should solve the problem
as well.

Is my understanding correct?

Thanks!

Oleg.