On Tue, Sep 09, 2014 at 11:42:35AM +0200, Peter Zijlstra wrote:
> On Fri, Aug 22, 2014 at 04:26:05PM +0300, Andreea-Cristina Bernat wrote:
> > The use of "rcu_assign_pointer()" is NULLing out the pointer.
> > According to RCU_INIT_POINTER()'s block comment:
> > "1. This use of RCU_INIT_POINTER() is NULLing out the pointer"
> > it is better to use it instead of rcu_assign_pointer() because it has a
> > smaller overhead.
> >
> > The following Coccinelle semantic patch was used:
> > @@
> > @@
> >
> > - rcu_assign_pointer
> > + RCU_INIT_POINTER
> > (..., NULL)
> >
> > Signed-off-by: Andreea-Cristina Bernat <bernat....@gmail.com>
> > ---
> > kernel/events/core.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 0d735be..4b84dd5 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -5744,7 +5744,7 @@ static void swevent_hlist_release(struct
> > swevent_htable *swhash)
> > if (!hlist)
> > return;
> >
> > - rcu_assign_pointer(swhash->swevent_hlist, NULL);
> > + RCU_INIT_POINTER(swhash->swevent_hlist, NULL);
> > kfree_rcu(hlist, rcu_head);
> > }
>
> Paul, why not do something like the below and do away with all this
> nonsense?
We used to do that, but the compilers became uncooperative, despite
Stephen Hemminger's best efforts.
Thanx, Paul
> ---
> include/linux/rcupdate.h | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index d231aa17b1d7..38c3e629ae70 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -623,7 +623,13 @@ 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) \
> +do { \
> + if (__builtin_constant_p(v) && v == NULL) \
> + p = RCU_INITIALIZER(v); \
> + else \
> + smp_store_release(&p, RCU_INITIALIZER(v)); \
> +} while (0)
>
> /**
> * rcu_access_pointer() - fetch RCU pointer with no dereferencing
--
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