From 765a3f4fed708ae429ee095914a7897acb3a65bd Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 14 Mar 2014 16:37:08 -0700 Subject: rcu: Provide grace-period piggybacking API The following pattern is currently not well supported by RCU: 1. Make data element inaccessible to RCU readers. 2. Do work that probably lasts for more than one grace period. 3. Do something to make sure RCU readers in flight before #1 above have completed. Here are some things that could currently be done: a. Do a synchronize_rcu() unconditionally at either #1 or #3 above. This works, but imposes needless work and latency. b. Post an RCU callback at #1 above that does a wakeup, then wait for the wakeup at #3. This works well, but likely results in an extra unneeded grace period. Open-coding this is also a bit more semi-tricky code than would be good. This commit therefore adds get_state_synchronize_rcu() and cond_synchronize_rcu() APIs. Call get_state_synchronize_rcu() at #1 above and pass its return value to cond_synchronize_rcu() at #3 above. This results in a call to synchronize_rcu() if no grace period has elapsed between #1 and #3, but requires only a load, comparison, and memory barrier if a full grace period did elapse. Requested-by: Peter Zijlstra Signed-off-by: Paul E. McKenney Acked-by: Peter Zijlstra --- include/linux/rcutree.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'include/linux/rcutree.h') diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h index e9c63884df0a..a59ca05fd4e3 100644 --- a/include/linux/rcutree.h +++ b/include/linux/rcutree.h @@ -76,6 +76,8 @@ static inline void synchronize_rcu_bh_expedited(void) void rcu_barrier(void); void rcu_barrier_bh(void); void rcu_barrier_sched(void); +unsigned long get_state_synchronize_rcu(void); +void cond_synchronize_rcu(unsigned long oldstate); extern unsigned long rcutorture_testseq; extern unsigned long rcutorture_vernum; -- cgit v1.2.3-70-g09d2