On Fri, Oct 09, 2020 at 11:30:38AM -0400, Qian Cai wrote: > On Fri, 2020-10-09 at 06:58 -0700, Paul E. McKenney wrote: > > On Fri, Oct 09, 2020 at 09:41:24AM -0400, Qian Cai wrote: > > > On Fri, 2020-10-09 at 07:58 +0000, tip-bot2 for Peter Zijlstra wrote: > > > > The following commit has been merged into the locking/core branch of tip: > > > > > > > > Commit-ID: 4d004099a668c41522242aa146a38cc4eb59cb1e > > > > Gitweb: > > > > https://git.kernel.org/tip/4d004099a668c41522242aa146a38cc4eb59cb1e > > > > Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > > > AuthorDate: Fri, 02 Oct 2020 11:04:21 +02:00 > > > > Committer: Ingo Molnar <mingo@xxxxxxxxxx> > > > > CommitterDate: Fri, 09 Oct 2020 08:53:30 +02:00 > > > > > > > > lockdep: Fix lockdep recursion > > > > > > > > Steve reported that lockdep_assert*irq*(), when nested inside lockdep > > > > itself, will trigger a false-positive. > > > > > > > > One example is the stack-trace code, as called from inside lockdep, > > > > triggering tracing, which in turn calls RCU, which then uses > > > > lockdep_assert_irqs_disabled(). > > > > > > > > Fixes: a21ee6055c30 ("lockdep: Change hardirq{s_enabled,_context} to per- > > > > cpu > > > > variables") > > > > Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx> > > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > > > > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> > > > > > > Reverting this linux-next commit fixed booting RCU-list warnings everywhere. > > > > Is it possible that the RCU-list warnings were being wrongly suppressed > > without a21ee6055c30? As in are you certain that these RCU-list warnings > > are in fact false positives? > > I guess you mean this commit a046a86082cc ("lockdep: Fix lockdep recursion") > instead of a21ee6055c30. It is unclear to me how that commit a046a86082cc would > suddenly start to generate those warnings, although I can see it starts to use > percpu variables even though the CPU is not yet set online. > > DECLARE_PER_CPU(unsigned int, lockdep_recursion); > > Anyway, the problem is that when we in the early boot: > > start_secondary() > smp_init_secondary() > init_cpu_timer() > clockevents_register_device() > > We are taking a lock there but the CPU is not yet online, and the > __lock_acquire() would call things like hlist_for_each_entry_rcu() from > lookup_chain_cache() or register_lock_class(). Thus, triggering the RCU-list > from an offline CPU warnings. > > I am not entirely sure how to fix those though. One approach is to move the call to rcu_cpu_starting() earlier in the start_secondary() processing. It is OK to invoke rcu_cpu_starting() multiple times, so for experiemental purposes you should be able to add a new call to it just before that lock is acquired. Thanx, Paul
