linux-kernel - Re: [PATCH tip/core/rcu 07/12] rcu: No ordering for rcu_assign

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Message-ID: <20160615221258.GE30927@twins.programming.kicks-ass.net>
Date:	Thu, 16 Jun 2016 00:12:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 07/12] rcu: No ordering for
 rcu_assign_pointer() of NULL

On Thu, Jun 16, 2016 at 12:03:39AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 15, 2016 at 02:46:08PM -0700, Paul E. McKenney wrote:
> > This commit does a compile-time check for rcu_assign_pointer() of NULL,
> > and uses WRITE_ONCE() rather than smp_store_release() in that case.
> > 
> > Reported-by: Christoph Hellwig <hch@...radead.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  include/linux/rcupdate.h | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index c61b6b9506e7..9be61e47badc 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -650,7 +650,16 @@ static inline void rcu_preempt_sleep_check(void)
> >   * please be careful when making changes to rcu_assign_pointer() and the
> >   * other macros that it invokes.
> >   */
> > -#define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))
> > +#define rcu_assign_pointer(p, v) \
> > +({ \
> > +	uintptr_t _r_a_p__v = (uintptr_t)(v); \
> > +	\
> > +	if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL) \
> > +		WRITE_ONCE((p), (typeof(p))(_r_a_p__v)); \
> > +	else \
> > +		smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
> > +	_r_a_p__v; \
> > +})
> 
> Can we pretty please right align the '\'s ?
> 
> Also, didn't we used to do this and then reverted it again for some
> obscure reason?


lkml.kernel.org/r/20140909094235.GD19379@...ns.programming.kicks-ass.net

What changed since then? And can we now pretty please get rid of that
RCU_INIT_POINTER() nonsense?