[PATCH 04/21] context_tracking: Add a note about noinstr VS unsafe context tracking functions

[Frederic Weisbecker]

Some context tracking functions enter or exit into/from RCU idle mode
while using trace-able and lockdep-aware IRQs (un-)masking. This could
be easily solved with using raw versions of local_irq_*() but we would
then lose some precious debugging informations.

Another possible way to solve this may consist in using rude RCU-tasks
in lockdep and irqsoff tracing.

In any case and until this get solved, those functions can't get
tagged as noinstr even though they should.

Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
Cc: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Nicolas Saenz Julienne <nsaenz@xxxxxxxxxx>
Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Cc: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
Cc: Yu Liao<liaoyu15@xxxxxxxxxx>
Cc: Phil Auld <pauld@xxxxxxxxxx>
Cc: Paul Gortmaker<paul.gortmaker@xxxxxxxxxxxxx>
Cc: Alex Belits <abelits@xxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 kernel/context_tracking.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 36a98c48aedc..1f44b0461d14 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -103,6 +103,11 @@ void noinstr __context_tracking_enter(enum ctx_state state)
 }
 EXPORT_SYMBOL_GPL(__context_tracking_enter);
 
+/*
+ * FIXME: This function should be noinstr but the below local_irq_restore() is
+ * unsafe because it involves illegal RCU uses through tracing and lockdep. This
+ * must be fixed first.
+ */
 void context_tracking_enter(enum ctx_state state)
 {
 	unsigned long flags;
@@ -125,6 +130,10 @@ void context_tracking_enter(enum ctx_state state)
 NOKPROBE_SYMBOL(context_tracking_enter);
 EXPORT_SYMBOL_GPL(context_tracking_enter);
 
+/*
+ * FIXME: This function should be noinstr but it unsafely calls local_irq_restore(),
+ * involving illegal RCU uses through tracing and lockdep. This must be fixed first.
+ */
 void context_tracking_user_enter(void)
 {
 	user_enter();
@@ -168,6 +177,11 @@ void noinstr __context_tracking_exit(enum ctx_state state)
 }
 EXPORT_SYMBOL_GPL(__context_tracking_exit);
 
+/*
+ * FIXME: This function should be noinstr but the below local_irq_save() is
+ * unsafe because it involves illegal RCU uses through tracing and lockdep. This
+ * must be fixed first.
+ */
 void context_tracking_exit(enum ctx_state state)
 {
 	unsigned long flags;
@@ -182,6 +196,10 @@ void context_tracking_exit(enum ctx_state state)
 NOKPROBE_SYMBOL(context_tracking_exit);
 EXPORT_SYMBOL_GPL(context_tracking_exit);
 
+/*
+ * FIXME: This function should be noinstr but it unsafely calls local_irq_save(),
+ * involving illegal RCU uses through tracing and lockdep. This must be fixed first.
+ */
 void context_tracking_user_exit(void)
 {
 	user_exit();
-- 
2.25.1