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

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

On Wed, Jun 28, 2006 at 11:41:21PM +0400, Oleg Nesterov wrote:
> 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.

Fortunately, we understood opposite sides of the problem, so, taken
together, we have it covered.  ;-)

Now we just need to figure out how to find the problems that both of
us missed!

> 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?

Exactly correct!!!

							Thanx, Paul