[RFC][PATCH v2 12/11] sched: Add preempt_count invariant check

[Peter Zijlstra]

Ingo requested I keep my debug check for the preempt_count invariant.

Requested-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 include/asm-generic/preempt.h |    2 +-
 include/linux/sched.h         |   17 +++++++++--------
 kernel/sched/core.c           |    5 +++++
 3 files changed, 15 insertions(+), 9 deletions(-)

--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -24,7 +24,7 @@ static __always_inline void preempt_coun
  * must be macros to avoid header recursion hell
  */
 #define init_task_preempt_count(p) do { \
-	task_thread_info(p)->preempt_count = 2*PREEMPT_DISABLED; \
+	task_thread_info(p)->preempt_count = INIT_PREEMPT_COUNT; \
 } while (0)
 
 #define init_idle_preempt_count(p, cpu) do { \
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -599,17 +599,18 @@ struct task_cputime_atomic {
 		.sum_exec_runtime = ATOMIC64_INIT(0),		\
 	}
 
-#ifdef CONFIG_PREEMPT_COUNT
-#define PREEMPT_DISABLED	(1 + PREEMPT_ENABLED)
-#else
-#define PREEMPT_DISABLED	PREEMPT_ENABLED
-#endif
+#define PREEMPT_DISABLED	(PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
 
 /*
- * Disable preemption until the scheduler is running.
- * Reset by start_kernel()->sched_init()->init_idle().
+ * Initial preempt_count value; reflects the preempt_count schedule invariant
+ * which states that during schedule preempt_count() == 2.
+ *
+ * This also results in the kernel starting with preemption disabled until
+ * the scheduler is initialized, see:
+ *
+ *   start_kernel()->sched_init()->init_idle().
  */
-#define INIT_PREEMPT_COUNT	PREEMPT_DISABLED
+#define INIT_PREEMPT_COUNT	(2*PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
 
 /**
  * struct thread_group_cputimer - thread group interval timer counts
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2503,6 +2503,11 @@ static struct rq *finish_task_switch(str
 	struct mm_struct *mm = rq->prev_mm;
 	long prev_state;
 
+	if (unlikely(WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
+			       "corrupted preempt_count: %s/%d/0x%x\n",
+			       current->comm, current->pid, preempt_count())))
+		preempt_count_set(INIT_PREEMPT_COUNT);
+
 	rq->prev_mm = NULL;
 
 	/*
--
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