[RFC PATCH 12/12] tracing: Pin recursion state for its complete lifetime

[Karl Mehltretter <kmehltretter@gmail.com>]

trace_test_and_set_recursion() publishes the task recursion bit before it
disables preemption. Scheduler tracing can therefore run while acquisition
is in progress. Conversely, trace_clear_recursion() enables preemption
before clearing the bit, so a reschedule can observe stale recursion state
during release.

CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING also validates RCU state before the
outer preemption disable. rcu_is_watching() drops its internal preemption
reference before returning, and the warning path accesses
TRACE_RECORD_RECURSION_BIT outside the protected lifetime.

Disable preemption before validation and retain that reference until the
recursion bit is cleared. Balance both rejection paths. This keeps
validation and published recursion state inside one preemption-disabled
interval.

Keep the transition slot unchanged. It remains necessary for
interrupt-entry intervals whose context is not yet reflected in
preempt_count().

Use preempt_enable_notrace() because this inline API is also used by
modules. The no-reschedule variant is unavailable there.

Assisted-by: LLM
---
 include/linux/trace_recursion.h | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index e6ca052b2a85a..fd14380751404 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h
@@ -126,11 +126,16 @@ extern void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip);
 static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsigned long pip,
 							int start)
 {
-	unsigned int val = READ_ONCE(current->trace_recursion);
+	unsigned int val;
 	int bit;
 
-	if (trace_warn_on_no_rcu(ip))
+	preempt_disable_notrace();
+	if (trace_warn_on_no_rcu(ip)) {
+		preempt_enable_notrace();
 		return -1;
+	}
+
+	val = READ_ONCE(current->trace_recursion);
 
 	bit = trace_get_context_bit() + start;
 	if (unlikely(val & (1 << bit))) {
@@ -145,6 +150,7 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign
 		bit = TRACE_CTX_TRANSITION + start;
 		if (val & (1 << bit)) {
 			do_ftrace_record_recursion(ip, pip);
+			preempt_enable_notrace();
 			return -1;
 		}
 	}
@@ -153,8 +159,6 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign
 	current->trace_recursion = val;
 	barrier();
 
-	preempt_disable_notrace();
-
 	return bit;
 }
 
@@ -163,9 +167,9 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign
  */
 static __always_inline void trace_clear_recursion(int bit)
 {
-	preempt_enable_notrace();
 	barrier();
 	trace_recursion_clear(bit);
+	preempt_enable_notrace();
 }
 
 /**
-- 
2.39.5 (Apple Git-154)