linux-kernel - Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup

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Message-ID: <20100416134336.GA2615@linux.vnet.ibm.com>
Date:	Fri, 16 Apr 2010 06:43:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast()
 implementation

On Fri, Apr 16, 2010 at 04:54:51PM +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2010-04-15 at 07:28 -0700, Paul E. McKenney wrote:
> > 
> > Of course, with call_rcu_sched(), the corresponding RCU read-side
> > critical
> > sections are non-preemptible.  Therefore, in CONFIG_PREEMPT_RT, these
> > read-side critical sections must use raw spinlocks.
> > 
> > Can the code in question accommodate these restrictions? 
> 
> What we protect against is always code that hard-disable IRQs (though
> there seem to be a bug in the hugepages code there...). Would that
> work ?

>From the perspective of call_rcu_sched() and synchronize_sched(),
the following things mark RCU-sched read-side critical sections:

1.	rcu_read_lock_sched() and rcu_read_unlock_sched().

2.	preempt_disable() and preempt_enable(), along with anything
	else that disables preemption.

3.	local_bh_disable() and local_bh_enable(), along with anything
	else that disables bottom halves.

4.	local_irq_disable() and local_irq_enable(), along wiht anything
	else that disables hardirqs.

5.	Handlers for NMIs.

So I believe that in this case call_rcu_sched() is your friend.  ;-)

							Thanx, Paul
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