On Thu, Apr 15, 2021 at 11:04:05PM +0800, Xu, Yanfei wrote: > Hi experts, > > I am learning rcu mechanism and its codes. When looking at the > rcu_blocking_is_gp(), I found there is a pair preemption disable/enable > operation in non-preemption code path. And it has been a long time. I can't > understand why we need it? Is there some thing I missed? If not, can we > remove the unnecessary operation like blow? Good point, you are right that preemption is disabled anyway in that block of code. However, preempt_disable() and preempt_enable() also prevent the compiler from moving that READ_ONCE() around. So my question to you is whether it is safe to remove those statements entirely or whether they should instead be replaced by barrier() or similar. Thanx, Paul > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index da6f5213fb74..c6d95a00715e 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3703,7 +3703,6 @@ static int rcu_blocking_is_gp(void) > if (IS_ENABLED(CONFIG_PREEMPTION)) > return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE; > might_sleep(); /* Check for RCU read-side critical section. */ > - preempt_disable(); > /* > * If the rcu_state.n_online_cpus counter is equal to one, > * there is only one CPU, and that CPU sees all prior accesses > @@ -3718,7 +3717,6 @@ static int rcu_blocking_is_gp(void) > * Those memory barriers are provided by CPU-hotplug code. > */ > ret = READ_ONCE(rcu_state.n_online_cpus) <= 1; > - preempt_enable(); > return ret; > } > > > > Best regards, > Yanfei
