sched/core: 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.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 6 insertions, 2 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index ea95ee1b5ef7..443677c8efe6 100644
--- 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)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6344d82a84f6..d6989f85c641 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2968,8 +2968,10 @@ static inline void schedule_debug(struct task_struct *prev)
         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));