On Mon, Jul 30 2018, Paul E. McKenney wrote: > On Tue, Jul 31, 2018 at 10:45:45AM +1000, NeilBrown wrote: >> On Fri, Jul 27 2018, Paul E. McKenney wrote: >> >> > On Thu, Jul 26, 2018 at 08:18:15PM -0700, Paul E. McKenney wrote: >> >> On Fri, Jul 27, 2018 at 11:04:37AM +1000, NeilBrown wrote: >> >> > On Wed, Jul 25 2018, Paul E. McKenney wrote: >> >> > >> >> >> > >> Looks good ... except ... naming is hard. >> >> > >> >> >> > >> is_after_call_rcu_init() asserts where in the lifecycle we are, >> >> > >> is_after_call_rcu() tests where in the lifecycle we are. >> >> > >> >> >> > >> The names are similar but the purpose is quite different. >> >> > >> Maybe s/is_after_call_rcu_init/call_rcu_init/ ?? >> >> > > >> >> > > How about rcu_head_init() and rcu_head_after_call_rcu()? >> >> >> >> Very well, I will pull this change in on my next rebase. >> > >> > Like this? >> >> Hard to say - unwinding white-space damage in my head is too challenging >> when newlines have been deleted :-( > > What??? Don't you like block-structured code? > > All kidding aside, how about the following more conventionally formatted > version? Wow - it's like I just got new glasses! Yes - nice an clear and now flaws to be found. Thanks a lot. NeilBrown > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit e3408141ed7d702995b2fdc94703af88aadd226b > Author: Paul E. McKenney > Date: Tue Jul 24 15:28:09 2018 -0700 > > rcu: Provide functions for determining if call_rcu() has been invoked > > This commit adds rcu_head_init() and rcu_head_after_call_rcu() functions > to help RCU users detect when another CPU has passed the specified > rcu_head structure and function to call_rcu(). The rcu_head_init() > should be invoked before making the structure visible to RCU readers, > and then the rcu_head_after_call_rcu() may be invoked from within > an RCU read-side critical section on an rcu_head structure that > was obtained during a traversal of the data structure in question. > The rcu_head_after_call_rcu() function will return true if the rcu_head > structure has already been passed (with the specified function) to > call_rcu(), otherwise it will return false. > > If rcu_head_init() has not been invoked on the rcu_head structure > or if the rcu_head (AKA callback) has already been invoked, then > rcu_head_after_call_rcu() will do WARN_ON_ONCE(). > > Reported-by: NeilBrown > Signed-off-by: Paul E. McKenney > [ paulmck: Apply neilb naming feedback. ] > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index e4f821165d0b..4db8bcacc51a 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -857,6 +857,46 @@ static inline notrace void rcu_read_unlock_sched_notrace(void) > #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */ > > > +/* Has the specified rcu_head structure been handed to call_rcu()? */ > + > +/* > + * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu() > + * @rhp: The rcu_head structure to initialize. > + * > + * If you intend to invoke rcu_head_after_call_rcu() to test whether a > + * given rcu_head structure has already been passed to call_rcu(), then > + * you must also invoke this rcu_head_init() function on it just after > + * allocating that structure. Calls to this function must not race with > + * calls to call_rcu(), rcu_head_after_call_rcu(), or callback invocation. > + */ > +static inline void rcu_head_init(struct rcu_head *rhp) > +{ > + rhp->func = (rcu_callback_t)~0L; > +} > + > +/* > + * rcu_head_after_call_rcu - Has this rcu_head been passed to call_rcu()? > + * @rhp: The rcu_head structure to test. > + * @func: The function passed to call_rcu() along with @rhp. > + * > + * Returns @true if the @rhp has been passed to call_rcu() with @func, > + * and @false otherwise. Emits a warning in any other case, including > + * the case where @rhp has already been invoked after a grace period. > + * Calls to this function must not race with callback invocation. One way > + * to avoid such races is to enclose the call to rcu_head_after_call_rcu() > + * in an RCU read-side critical section that includes a read-side fetch > + * of the pointer to the structure containing @rhp. > + */ > +static inline bool > +rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f) > +{ > + if (READ_ONCE(rhp->func) == f) > + return true; > + WARN_ON_ONCE(READ_ONCE(rhp->func) != (rcu_callback_t)~0L); > + return false; > +} > + > + > /* Transitional pre-consolidation compatibility definitions. */ > > static inline void synchronize_rcu_bh(void) > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > index 5dec94509a7e..4c56c1d98fb3 100644 > --- a/kernel/rcu/rcu.h > +++ b/kernel/rcu/rcu.h > @@ -224,6 +224,7 @@ void kfree(const void *); > */ > static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head) > { > + rcu_callback_t f; > unsigned long offset = (unsigned long)head->func; > > rcu_lock_acquire(&rcu_callback_map); > @@ -234,7 +235,9 @@ static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head) > return true; > } else { > RCU_TRACE(trace_rcu_invoke_callback(rn, head);) > - head->func(head); > + f = head->func; > + WRITE_ONCE(head->func, (rcu_callback_t)0L); > + f(head); > rcu_lock_release(&rcu_callback_map); > return false; > }