On Tue, Sep 01, 2020 at 08:51:46PM -0700, Guenter Roeck wrote:
> On Fri, Aug 21, 2020 at 10:47:49AM +0200, Peter Zijlstra wrote:
> > The lockdep tracepoints are under the lockdep recursion counter, this
> > has a bunch of nasty side effects:
> >
> > - TRACE_IRQFLAGS doesn't work across the entire tracepoint
> >
> > - RCU-lockdep doesn't see the tracepoints either, hiding numerous
> > "suspicious RCU usage" warnings.
> >
> > Pull the trace_lock_*() tracepoints completely out from under the
> > lockdep recursion handling and completely rely on the trace level
> > recusion handling -- also, tracing *SHOULD* not be taking locks in any
> > case.
> >
>
> Wonder what is worse - the problem or its fix. This patch results in
> a number of WARNING backtraces for several archtectures/platforms.
> Reverting it fixes the problems.
Without all this there was a recursion that could crash. But yes,
tedious.
OTOH the warnings are about real bugs that were pre-existing, we now see
them and can fix them.
I'll reply to ARM separately, but let's have a peek at s390.
> s390:
>
> [ 19.490586] =============================
> [ 19.490752] WARNING: suspicious RCU usage
> [ 19.490921] 5.9.0-rc3 #1 Not tainted
> [ 19.491086] -----------------------------
> [ 19.491253] include/trace/events/lock.h:37 suspicious
> rcu_dereference_check() usage!
> [ 19.493147] [<00000000001d5de2>] lock_acquire+0x41a/0x498
> [ 19.493320] [<0000000000103b72>] enabled_wait+0xca/0x198
> [ 19.493493] [<0000000000103f80>] arch_cpu_idle+0x20/0x38
Does this help?
---
diff --git a/arch/s390/kernel/idle.c b/arch/s390/kernel/idle.c
index c73f50649e7e..f7f1e64e0d98 100644
--- a/arch/s390/kernel/idle.c
+++ b/arch/s390/kernel/idle.c
@@ -39,14 +39,13 @@ void enabled_wait(void)
local_irq_restore(flags);
/* Account time spent with enabled wait psw loaded as idle time. */
- /* XXX seqcount has tracepoints that require RCU */
- write_seqcount_begin(&idle->seqcount);
+ raw_write_seqcount_begin(&idle->seqcount);
idle_time = idle->clock_idle_exit - idle->clock_idle_enter;
idle->clock_idle_enter = idle->clock_idle_exit = 0ULL;
idle->idle_time += idle_time;
idle->idle_count++;
account_idle_time(cputime_to_nsecs(idle_time));
- write_seqcount_end(&idle->seqcount);
+ raw_write_seqcount_end(&idle->seqcount);
}
NOKPROBE_SYMBOL(enabled_wait);
