On Thu, Jun 16, 2016 at 12:12:58AM +0200, Peter Zijlstra wrote: > 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 <[email protected]> > > > Signed-off-by: Paul E. McKenney <[email protected]> > > > --- > > > 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 ?
If you insist... ;-) Done. > > Also, didn't we used to do this and then reverted it again for some > > obscure reason? > > lkml.kernel.org/r/[email protected] There was indeed a compiler bug long ago that could generate spurious warnings: https://groups.google.com/forum/#!topic/linux.kernel/y2FIhJ-WVJc > What changed since then? And can we now pretty please get rid of that > RCU_INIT_POINTER() nonsense? Five years has passed, the structure of rcu_assign_pointer() has completely changed, and someone asked for the old behavior. Seemed worth a try, given the very visible nature of the gcc complaint. No complaints thus far, but then again there probably aren't that many people running -rcu. That said, I am encouraged by the lack of reports from the 0day test robot. If this goes in and there aren't any problems for some time, then I agree that shrinking the RCU API would be worthwhile. My idea of "some time" is about a year, given that it would be a real pain to push a bunch of changes throughout the kernel only to have to revert them if the old compiler bug managed to crop up again. :-/ Thanx, Paul
