linux-kernel - Re: [PATCH 2/2] percpu-rw-semaphores: use rcu_read_lock

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Date:	Wed, 24 Oct 2012 20:43:11 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 2/2] percpu-rw-semaphores: use rcu_read_lock_sched

On 10/24, Paul E. McKenney wrote:
>
> On Wed, Oct 24, 2012 at 07:18:55PM +0200, Oleg Nesterov wrote:
> > On 10/24, Paul E. McKenney wrote:
> > >
> > > static inline void percpu_up_read(struct percpu_rw_semaphore *p)
> > > {
> > > 	/*
> > > 	 * Decrement our count, but protected by RCU-sched so that
> > > 	 * the writer can force proper serialization.
> > > 	 */
> > > 	rcu_read_lock_sched();
> > > 	this_cpu_dec(*p->counters);
> > > 	rcu_read_unlock_sched();
> > > }
> >
> > Yes, the explicit lock/unlock makes the new assumptions about
> > synchronize_sched && barriers unnecessary. And iiuc this could
> > even written as
> >
> > 	rcu_read_lock_sched();
> > 	rcu_read_unlock_sched();
> >
> > 	this_cpu_dec(*p->counters);
>
> But this would lose the memory barrier that is inserted by
> synchronize_sched() after the CPU's last RCU-sched read-side critical
> section.

How? Afaics there is no need to synchronize with this_cpu_dec(), its
result was already seen before the 2nd synchronize_sched() was called
in percpu_down_write().

IOW, this memory barrier is only needed to synchronize with memory
changes inside down_read/up_read.

To clarify, of course I do not suggest to write is this way. I am just
trying to check my understanding.

Oleg.

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