linux-kernel - Re: [PATCH RFC tip/core/rcu 1/5] rcu: Reduce overhead of cond

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Date:	Mon, 23 Jun 2014 17:18:51 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	sbw@....edu, Andi Kleen <ak@...ux.intel.com>,
	Christoph Lameter <cl@...two.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH RFC tip/core/rcu 1/5] rcu: Reduce overhead of
 cond_resched() checks for RCU

On Mon, Jun 23, 2014 at 08:35:27PM +0200, Oleg Nesterov wrote:
> On 06/23, Paul E. McKenney wrote:
> >
> > On Mon, Jun 23, 2014 at 06:43:21PM +0200, Oleg Nesterov wrote:
> > >
> > > should equally work, or ACCESS_ONCE() can't be used to RMW ?
> >
> > It can be, but Linus doesn't like it to be.  I recently changed all of
> > the RMW ACCESS_ONCE() calls as a result.  One of the reasons for avoiding
> > RMW ACCESS_ONCE() is that language features that might one day replace
> > ACCESS_ONCE() do not support RMW use.
> 
> OK, thanks.
> 
> > > Or even INIT_LIST_HEAD_RCU(). The comment in list_splice_init_rcu() says:
> > >
> > > 	/*
> > > 	 * "first" and "last" tracking list, so initialize it.  RCU readers
> > > 	 * have access to this list, so we must use INIT_LIST_HEAD_RCU()
> > > 	 * instead of INIT_LIST_HEAD().
> > > 	 */
> > >
> > > 	INIT_LIST_HEAD_RCU(list);
> > >
> > > but we are going to call synchronize_rcu() or something similar, this should
> > > act as compiler barrier too?
> >
> > Indeed, synchronize_rcu() enforces a barrier on each CPU between
> > any prior and subsequent accesses to RCU-protected data by that CPU.
> > (Which means that CPUs that would otherwise sleep through the entire
> > grace period can continue sleeping, given that it is not accessing
> > any RCU-protected data while sleeping.)  I would guess load-tearing
> > or store-tearing concerns.
> 
> But the kernel depends on the fact that "long" should be updated atomically,
> and the concurent reader should see the old-or-new value without any tricks.
> 
> Perhaps we should add ACCESS_ONCE_PARANOID_FOR_COMPILER(). Otherwise when
> you read the code it is not always clear why it is uses ACCESS_ONCE(), and
> sometimes this look as if you simply do not understand it. Or at least a
> /* not really needed but gcc can have bugs */ could help in these cases.

I am a bit reluctant to add variants of ACCESS_ONCE(), but perhaps
comments about exactly what the ACCESS_ONCE() is preventing in the more
paranoid cases would be a good thing.  My fear is that the comments will
just be copy/pasted with the ACCESS_ONCE wrappers.  ;-)

							Thanx, Paul

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