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[PATCH v2 07/12] sched: Robustify preemption leak checks

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  [PATCH v2 07/12] sched: Robustify preemption leak checks Peter Zijlstra <peterz@infradead.org> - 2015-09-30 09:40 +0200
    Re: [PATCH v2 07/12] sched: Robustify preemption leak checks Steven Rostedt <rostedt@goodmis.org> - 2015-09-30 11:40 +0200

#1235842 — [PATCH v2 07/12] sched: Robustify preemption leak checks

When we warn about a preempt_count leak; reset the preempt_count to
the known good value such that the problem does not ripple forward.

This is most important on x86 which has a per cpu preempt_count that is
not saved/restored (after this series). So if you schedule with an
invalid (!2*PREEMPT_DISABLE_OFFSET) preempt_count the next task is
messed up too.

Enforcing this invariant limits the borkage to just the one task.

Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/exit.c       |    4 +++-
 kernel/sched/core.c |    4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -706,10 +706,12 @@ void do_exit(long code)
 	smp_mb();
 	raw_spin_unlock_wait(&tsk->pi_lock);
 
-	if (unlikely(in_atomic()))
+	if (unlikely(in_atomic())) {
 		pr_info("note: %s[%d] exited with preempt_count %d\n",
 			current->comm, task_pid_nr(current),
 			preempt_count());
+		preempt_count_set(PREEMPT_ENABLED);
+	}
 
 	/* sync mm's RSS info before statistics gathering */
 	if (tsk->mm)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2968,8 +2968,10 @@ static inline void schedule_debug(struct
 	BUG_ON(unlikely(task_stack_end_corrupted(prev)));
 #endif
 
-	if (unlikely(in_atomic_preempt_off()))
+	if (unlikely(in_atomic_preempt_off())) {
 		__schedule_bug(prev);
+		preempt_count_set(PREEMPT_DISABLED);
+	}
 	rcu_sleep_check();
 
 	profile_hit(SCHED_PROFILING, __builtin_return_address(0));


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#1235944

On Wed, 30 Sep 2015 09:10:42 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> When we warn about a preempt_count leak; reset the preempt_count to
> the known good value such that the problem does not ripple forward.
> 
> This is most important on x86 which has a per cpu preempt_count that is
> not saved/restored (after this series). So if you schedule with an
> invalid (!2*PREEMPT_DISABLE_OFFSET) preempt_count the next task is
> messed up too.
> 
> Enforcing this invariant limits the borkage to just the one task.
> 
> Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Reviewed-by: Steven Rostedt <rostedt@goodmis.org>

-- Steve

> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---

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