linux-kernel - Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

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Date:	Tue, 23 Oct 2012 13:32:54 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

On Tue, Oct 23, 2012 at 01:29:02PM -0700, Paul E. McKenney wrote:
> On Tue, Oct 23, 2012 at 08:41:23PM +0200, Oleg Nesterov wrote:
> > On 10/23, Paul E. McKenney wrote:
> > >
> > >  * Note that this guarantee implies a further memory-ordering guarantee.
> > >  * On systems with more than one CPU, when synchronize_sched() returns,
> > >  * each CPU is guaranteed to have executed a full memory barrier since
> > >  * the end of its last RCU read-side critical section
> >          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > 
> > Ah wait... I misread this comment.
> 
> And I miswrote it.  It should say "since the end of its last RCU-sched
> read-side critical section."  So, for example, RCU-sched need not force
> a CPU that is idle, offline, or (eventually) executing in user mode to
> execute a memory barrier.  Fixed this.

And I should hasten to add that for synchronize_sched(), disabling
preemption (including disabling irqs, further including NMI handlers)
acts as an RCU-sched read-side critical section.  (This is in the
comment header for synchronize_sched() up above my addition to it.)
	
							Thanx, Paul

> > But this patch needs more? Or I misunderstood. There is no RCU unlock
> > in percpu_up_read().
> > 
> > IOW. Suppose the code does
> > 
> > 	percpu_down_read();
> > 	x = PROTECTED_BY_THIS_RW_SEM;
> > 	percpu_up_read();
> > 
> > Withoit mb() the load above can be reordered with this_cpu_dec() in
> > percpu_up_read().
> > 
> > However, we do not care if we can guarantee that the next
> > percpu_down_write() can not return (iow, the next "write" section can
> > not start) until this load is complete.
> > 
> > And I _think_ that another synchronize_sched() in percpu_down_write()
> > added by this patch should work.
> > 
> > But, "since the end of its last  RCU read-side critical section"
> > does not look enough.
> > 
> > Or I misundersood you/Mikulas/both ?
> 
> I clearly need to look more carefully at Mikulas's code...
> 
> 							Thanx, Paul

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