linux-kernel - Re: [PATCH 0/6] Switch __DECLARE_TRACE() to new notrace variant of SRCU-fast

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Message-ID: <20250723163450.02bc299b@gandalf.local.home>
Date: Wed, 23 Jul 2025 16:34:50 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH 0/6] Switch __DECLARE_TRACE() to new notrace variant of
 SRCU-fast

On Wed, 23 Jul 2025 13:27:54 -0700
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> This triggers continues to trigger a kernel test robot report of a
> "using smp_processor_id() in preemptible" splat.  I looked for issues
> with explicit preemption disabling, and, not finding any, will next turn
> my attention to accesses to per-CPU variables.  Any and all insights
> are welcome.

Currently perf and ftrace expect the tracepoints to be called with
preemption disabled. You may need this:

diff --git a/include/trace/perf.h b/include/trace/perf.h
index a1754b73a8f5..1b7925a85966 100644
--- a/include/trace/perf.h
+++ b/include/trace/perf.h
@@ -71,7 +71,9 @@ perf_trace_##call(void *__data, proto)					\
 	u64 __count __attribute__((unused));				\
 	struct task_struct *__task __attribute__((unused));		\
 									\
+	preempt_disable_notrace();					\
 	do_perf_trace_##call(__data, args);				\
+	preempt_enable_notrace();					\
 }
 
 #undef DECLARE_EVENT_SYSCALL_CLASS
diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
index 4f22136fd465..0504a423ca25 100644
--- a/include/trace/trace_events.h
+++ b/include/trace/trace_events.h
@@ -436,7 +436,9 @@ __DECLARE_EVENT_CLASS(call, PARAMS(proto), PARAMS(args), PARAMS(tstruct), \
 static notrace void							\
 trace_event_raw_event_##call(void *__data, proto)			\
 {									\
+	preempt_disable_notrace();					\
 	do_trace_event_raw_event_##call(__data, args);			\
+	preempt_enable_notrace();					\
 }
 
 #undef DECLARE_EVENT_SYSCALL_CLASS


But please add it with the change, as there's "preempt_count" accounting to
report to the user that accounts that preemption was disabled when called.

-- Steve