On Tue, 14 Mar 2023 14:08:28 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > On Tue, Mar 14, 2023 at 03:02:41PM -0400, Steven Rostedt wrote: > > From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx> > > > > A while ago where the trace events had the following: > > > > rcu_read_lock_sched_notrace(); > > rcu_dereference_sched(...); > > rcu_read_unlock_sched_notrace(); > > > > If the tracepoint is enabled, it could trigger RCU issues if called in > > the wrong place. And this warning was only triggered if lockdep was > > enabled. If the tracepoint was never enabled with lockdep, the bug would > > not be caught. To handle this, the above sequence was done when lockdep > > was enabled regardless if the tracepoint was enabled or not (although the > > always enabled code really didn't do anything, it would still trigger a > > warning). > > > > But a lot has changed since that lockdep code was added. One is, that > > sequence no longer triggers any warning. Another is, the tracepoint when > > enabled doesn't even do that sequence anymore. > > > > The main check we care about today is whether RCU is "watching" or not. > > So if lockdep is enabled, always check if rcu_is_watching() which will > > trigger a warning if it is not (tracepoints require RCU to be watching). > > > > Note, that old sequence did add a bit of overhead when lockdep was enabled, > > and with the latest kernel updates, would cause the system to slow down > > enough to trigger kernel "stalled" warnings. > > > > Link: http://lore.kernel.org/lkml/20140806181801.GA4605@xxxxxxxxxx > > Link: http://lore.kernel.org/lkml/20140807175204.C257CAC5@xxxxxxxxxxxxxxxxxx > > Link: https://lore.kernel.org/lkml/20230307184645.521db5c9@xxxxxxxxxxxxxxxxxx/ > > Link: https://lore.kernel.org/linux-trace-kernel/20230310172856.77406446@xxxxxxxxxxxxxxxxxx > > > > Cc: stable@xxxxxxxxxxxxxxx > > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx> > > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> > > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> > > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> > > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > > Fixes: e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use SRCU") > > Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx> > > Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > Thanks Paul! -- Steve
