[PATCH RFC v4 12/13] rcu-tasks-trace: Assert no reader is held on return to userspace

[Josef Bacik]

With trampolines now taking rcu_read_lock_trace() open-coded from
assembly and JIT-emitted code, an unbalanced reader would silently
turn every later Tasks Trace grace period on that task into a stall.
No task can legitimately reach userspace with current->trc_reader_nesting
non-zero, so under CONFIG_PROVE_RCU check it in the generic entry
code's return-to-user validation, next to the existing kmap and lockdep
assertions.  Compiles away otherwise.

Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 include/linux/irq-entry-common.h | 2 ++
 include/linux/rcupdate_trace.h   | 7 +++++++
 2 files changed, 9 insertions(+)

diff --git a/include/linux/irq-entry-common.h b/include/linux/irq-entry-common.h
index e19b41ee6b18..d42568a488a7 100644
--- a/include/linux/irq-entry-common.h
+++ b/include/linux/irq-entry-common.h
@@ -5,6 +5,7 @@
 #include <linux/context_tracking.h>
 #include <linux/hrtimer_rearm.h>
 #include <linux/kmsan.h>
+#include <linux/rcupdate_trace.h>
 #include <linux/rseq_entry.h>
 #include <linux/static_call_types.h>
 #include <linux/syscalls.h>
@@ -214,6 +215,7 @@ static __always_inline void __exit_to_user_mode_validate(void)
 {
 	/* Ensure that kernel state is sane for a return to userspace */
 	kmap_assert_nomap();
+	rcu_tasks_trace_assert_idle();
 	lockdep_assert_irqs_disabled();
 	lockdep_sys_exit();
 }
diff --git a/include/linux/rcupdate_trace.h b/include/linux/rcupdate_trace.h
index 273c59a03251..6034c6b19479 100644
--- a/include/linux/rcupdate_trace.h
+++ b/include/linux/rcupdate_trace.h
@@ -211,6 +211,12 @@ unsigned long rcu_tasks_trace_batches_completed(void);
 // Placeholders to enable stepwise transition.
 void __init rcu_tasks_trace_suppress_unused(void);
 
+/* A task must never reach userspace inside an rcu_read_lock_trace() reader. */
+static inline void rcu_tasks_trace_assert_idle(void)
+{
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && READ_ONCE(current->trc_reader_nesting));
+}
+
 #else
 static inline unsigned long rcu_tasks_trace_batches_completed(void) { return 0; }
 /*
@@ -220,6 +226,7 @@ static inline unsigned long rcu_tasks_trace_batches_completed(void) { return 0;
 static inline void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func) { BUG(); }
 static inline void rcu_read_lock_trace(void) { BUG(); }
 static inline void rcu_read_unlock_trace(void) { BUG(); }
+static inline void rcu_tasks_trace_assert_idle(void) { }
 #endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
 
 DEFINE_LOCK_GUARD_0(rcu_tasks_trace,

-- 
2.55.0