On Wed, Oct 14, 2020 at 11:53:19PM +0200, Peter Zijlstra wrote: > On Wed, Oct 14, 2020 at 11:34:05AM -0700, Paul E. McKenney wrote: > > commit 7deaa04b02298001426730ed0e6214ac20d1a1c1 > > Author: Paul E. McKenney <paulmck@xxxxxxxxxx> > > Date: Tue Oct 13 12:39:23 2020 -0700 > > > > rcu: Prevent lockdep-RCU splats on lock acquisition/release > > > > The rcu_cpu_starting() and rcu_report_dead() functions transition the > > current CPU between online and offline state from an RCU perspective. > > Unfortunately, this means that the rcu_cpu_starting() function's lock > > acquisition and the rcu_report_dead() function's lock releases happen > > while the CPU is offline from an RCU perspective, which can result in > > lockdep-RCU splats about using RCU from an offline CPU. In reality, > > aside from the splats, both transitions are safe because a new grace > > period cannot start until these functions release their locks. > > But we call the trace_* crud before we acquire the lock. Are you sure > that's a false-positive? You lost me on this one. I am assuming that you are talking about rcu_cpu_starting(), because that is the one where RCU is not initially watching, that is, the case where tracing before the lock acquisition would be a problem. You cannot be talking about rcu_cpu_starting() itself, because it does not do any tracing before acquiring the lock. But if you are talking about the caller of rcu_cpu_starting(), then that caller should put the rcu_cpu_starting() before the tracing. But that would be the other patch earlier in this thread that was proposing moving the call to rcu_cpu_starting() much earlier in CPU bringup. So what am I missing here? Thanx, Paul
