On 2021-09-22 13:38:20 [+0200], Frederic Weisbecker wrote: > > The part with rcutree.use_softirq=0 on RT does not make it any better, > > right? > > The rcuc kthread disables softirqs before calling rcu_core(), so it behaves > pretty much the same as a softirq. Or am I missing something? Oh, no you don't. > > So you rely on some implicit behaviour which breaks with RT such as: > > > > CPU 0 > > ----------------------------------------------- > > RANDOM TASK-A RANDOM TASK-B > > ------ ----------- > > int *X = &per_cpu(CPUX, 0) int *X = &per_cpu(CPUX, 0) > > int A, B; > > spin_lock(&D); > > spin_lock(&C); > > WRITE_ONCE(*X, 0); > > A = READ_ONCE(*X); > > WRITE_ONCE(*X, 1); > > B = READ_ONCE(*X); > > > > while spinlock C and D are just random locks not related to CPUX but it > > just happens that they are held at that time. So for !RT you guarantee > > that A == B while it is not the case on RT. > > Not sure which spinlocks you are referring to here. Also most RCU spinlocks > are raw. I was bringing an example where you also could rely on implicit locking provided by spin_lock() which breaks on RT. Sebastian
