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

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Message-ID: <20100415142852.GA2471@linux.vnet.ibm.com>
Date:	Thu, 15 Apr 2010 07:28:52 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.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 Wed, Apr 14, 2010 at 03:51:50PM +0200, Peter Zijlstra wrote:
> On Mon, 2010-04-12 at 20:43 -0700, Paul E. McKenney wrote:
> > > So we might have to support the interrupt assumption, at least in some
> > > form, with those guys...
> > 
> > One way to make the interrupt assumption official is to use
> > synchronize_sched() rather than synchronize_rcu().
> 
> Well, call_rcu_sched() then, because the current usage is to use
> call_rcu() to free the page directories.
> 
> Paul, here is a call_rcu_sched() available in kernel/rcutree.c, but am I
> right in reading that code that that would not be available for
> preemptible RCU?

Both call_rcu_sched() and call_rcu() are always there for you.  ;-)

o	If CONFIG_TREE_RCU (or CONFIG_TINY_RCU), they both have the same
	implementation.

o	If CONFIG_TREE_PREEMPT_RCU, call_rcu_sched() is preemptible and
	call_rcu() is not.

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?

							Thanx, Paul
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