On Mon, 28 Sep 2026 17:41:57 -0700 "Paul E. McKenney" <[email protected]> wrote:
> > > > Thanks for the clarification! > > > > So I think we can replace rcu_read_lock() with > > > > rcu_read_lock_sched_notrace()? > > > > (also, maybe we need to add rcu_is_watching() check) > > > > > > That would work, and yes, you are quite right that if !rcu_is_watching(), > > > RCU is studiously ignoring you completely, no matter what your preemption > > > state is. > > > > > > > Current code may involves lockdep in fprobe, that is not good. > > > > > > Agreed, that could be problematic in any number of contexts. ;-) > > > > > > > BTW, if we want to trace some code in extended quiescent state, do we > > > > still need this Tasks-Rude RCU? > > > > > > Well, you need something other than vanilla RCU, anyway. And Tasks Rude > > > RCU is one candidate, though it is also a candidate that might annoy > > > people wishing to minimize IPIs. > > > > Would RCU Tasks Trace be a better candidate given uprobes and > > sleepable BPF depends on it? Would it work here? Hmm, those can be sleepable, but this is not. fprobe is based on ftrace so the callbacks are non preemptive. > > Quite possibly! Of course, there might be corner-case ftrace/BPF issues > that you and Masami would know better than would I. > > David Woodhouse suggests the new Atomic SRCU for a vaguely similar > purpose: > > https://lore.kernel.org/all/[email protected]/ > > Now that Tasks Trace RCU is implemented in terms of SRCU-fast, there > might not be all that much air between David's suggestion and yours. OK, let me check it. But at a glance, this atomic SRCU is still a bit heavier than rcu_sched_notrace, for fprobe. It involves smp_mb and lockdep. So if all of these rcu_read_lock() in fprobe are replaced with rcu_read_lock_sched_notrace(), isn't it enough to switch the synchronize_rcu() to synchronize_rcu_tasks_rude() for avoiding `if (!rcu_is_watching()) return;` ? (Note, most of replacement will be done by [1]) [1] https://lore.kernel.org/all/179064115227.394389.16910234241400391996.stgit@devnote2/ Thank you, -- Masami Hiramatsu (Google) <[email protected]>
