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

From: "Paul E. McKenney" <[email-masked]>
Date: Sun, 23 Mar 2014 15:12:56 -0700

On Sun, Mar 23, 2014 at 11:33:49PM +0200, Michael S. Tsirkin wrote:
> 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.

This would at the very least need to be tested under a wide variety
of compilers.  And we need to keep 

> 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)) \

You lost me on this one.  If "v" is not a built-in constant, we want
the smp_wmb(), right?

							Thanx, Paul

> +			smp_wmb(); \
>  		ACCESS_ONCE(p) = RCU_INITIALIZER(v); \
>  	} while (0)
> 
>