sched/core: Allow try_invoke_on_locked_down_task() with irqs disabled

["Paul E. McKenney" <paulmck@kernel.org>]

Hello, Peter!

Are you OK with your Signed-off-by on this patch?

Or is there some other approach you would prefer?

							Thanx, Paul

------------------------------------------------------------------------

commit 903c5302fa2df20c4074ea1fef01b1be9b4065d7
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Sat Aug 29 10:22:24 2020 -0700

    sched/core: Allow try_invoke_on_locked_down_task() with irqs disabled
    
    The try_invoke_on_locked_down_task() function currently requires
    that interrupts be enabled, but it is called with interrupts
    disabled from rcu_print_task_stall(), resulting in an "IRQs not
    enabled as expected" diagnostic.  This commit therefore updates
    try_invoke_on_locked_down_task() to use raw_spin_lock_irqsave() instead
    of raw_spin_lock_irq(), thus allowing use from either context.
    
    Link: https://lore.kernel.org/lkml/000000000000903d5805ab908fc4@google.com/
    Reported-by: syzbot+cb3b69ae80afd6535b0e@syzkaller.appspotmail.com
    Not-signed-off-by: Peter Zijlstra <peterz@infradead.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8471a0f..a814028 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2988,7 +2988,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 
 /**
  * try_invoke_on_locked_down_task - Invoke a function on task in fixed state
- * @p: Process for which the function is to be invoked.
+ * @p: Process for which the function is to be invoked, can be @current.
  * @func: Function to invoke.
  * @arg: Argument to function.
  *
@@ -3006,12 +3006,11 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
  */
 bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct task_struct *t, void *arg), void *arg)
 {
-	bool ret = false;
 	struct rq_flags rf;
+	bool ret = false;
 	struct rq *rq;
 
-	lockdep_assert_irqs_enabled();
-	raw_spin_lock_irq(&p->pi_lock);
+	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
 	if (p->on_rq) {
 		rq = __task_rq_lock(p, &rf);
 		if (task_rq(p) == rq)
@@ -3028,7 +3027,7 @@ bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct t
 				ret = func(p, arg);
 		}
 	}
-	raw_spin_unlock_irq(&p->pi_lock);
+	raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);
 	return ret;
 }