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Subject[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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