Subject: Re: [PATCH] drivers/net: Use RCU_INIT_POINTER(x, NULL) in tun.c

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 23 Mar 2014 23:33:49 +0200

On Sun, Mar 23, 2014 at 12:54:17PM -0700, Eric Dumazet wrote:
> On Sun, 2014-03-23 at 21:41 +0200, Michael S. Tsirkin wrote:
> 
> > The rcu_assign_pointer() ensures that the initialization of a structure       
> > is carried out before storing a pointer to that structure. 
> > In the case of the NULL pointer, there is no structure to initialize,
> > so we can safely drop smp_wmb in this case.
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > 
> > --
> > 
> > Lightly tested.
> > v is evaluated twice here but that should be ok since this
> > only happens when v is a constant, so evaluating it should
> > have no side effects.
> > Paul, what do you think?
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 72bf3a0..d33c9ec 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -587,7 +587,8 @@ static inline void rcu_preempt_sleep_check(void)
> >   */
> >  #define rcu_assign_pointer(p, v) \
> >  	do { \
> > -		smp_wmb(); \
> > +		if (!__builtin_constant_p(v) || (v)) \
> > +			smp_wmb(); \
> >  		ACCESS_ONCE(p) = RCU_INITIALIZER(v); \
> >  	} while (0)
> >  
> 
> Yeah, I suggest you read d322f45ceed525daa changelog ;)
> 

Oh I see. It does not seem hard to silence that warning though.
See below.
Alternatively apply these patches everywhere though it does
look like too much work for too little gain to me.

-->

rcu: optimize rcu_assign_pointer with NULL

The rcu_assign_pointer() dropped __builtin_constant_p check to
avoid a compiler warning, but we can actually work around it without
adding code.

Signed-off-by: Michael S. Tsirkin <[email-masked]>

---

Untested, too late here, sorry.

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 72bf3a0..9111d40 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -585,9 +585,14 @@ 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.
  */
+/* The convoluted __builtin_constant_p logic is here to prevent
+ * gcc from emitting a warning when passed a pointer to a variable.
+ */
 #define rcu_assign_pointer(p, v) \
 	do { \
-		smp_wmb(); \
+		if (!__builtin_constant_p(v) || \
+		    (__builtin_constant_p(v) ? (v) : NULL)) \
+			smp_wmb(); \
 		ACCESS_ONCE(p) = RCU_INITIALIZER(v); \
 	} while (0)