sched: Consider task_struct::saved_state in wait_task_inactive()

With the introduction of task_struct::saved_state in commit
5f220be21418 ("sched/wakeup: Prepare for RT sleeping spin/rwlocks")
matching the task state has gotten more complicated. That same commit
changed try_to_wake_up() to consider both states, but
wait_task_inactive() has been neglected.

Sebastian noted that the wait_task_inactive() usage in
ptrace_check_attach() can misbehave when ptrace_stop() is blocked on
the tasklist_lock after it sets TASK_TRACED.

Therefore extract a common helper from ttwu_state_match() and use that
to teach wait_task_inactive() about the PREEMPT_RT locks.

Originally-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20230601091234.GW83892@hirez.programming.kicks-ass.net

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 810cf7d..ac38225 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2213,6 +2213,39 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
                rq_clock_skip_update(rq);
 }
 
+static __always_inline
+int __task_state_match(struct task_struct *p, unsigned int state)
+{
+       if (READ_ONCE(p->__state) & state)
+               return 1;
+
+#ifdef CONFIG_PREEMPT_RT
+       if (READ_ONCE(p->saved_state) & state)
+               return -1;
+#endif
+       return 0;
+}
+
+static __always_inline
+int task_state_match(struct task_struct *p, unsigned int state)
+{
+#ifdef CONFIG_PREEMPT_RT
+       int match;
+
+       /*
+        * Serialize against current_save_and_set_rtlock_wait_state() and
+        * current_restore_rtlock_saved_state().
+        */
+       raw_spin_lock_irq(&p->pi_lock);
+       match = __task_state_match(p, state);
+       raw_spin_unlock_irq(&p->pi_lock);
+
+       return match;
+#else
+       return __task_state_match(p, state);
+#endif
+}
+
 /*
  * wait_task_inactive - wait for a thread to unschedule.
  *
@@ -2231,7 +2264,7 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
  */
 unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state)
 {
-       int running, queued;
+       int running, queued, match;
        struct rq_flags rf;
        unsigned long ncsw;
        struct rq *rq;
@@ -2257,7 +2290,7 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
                 * is actually now running somewhere else!
                 */
                while (task_on_cpu(rq, p)) {
-                       if (!(READ_ONCE(p->__state) & match_state))
+                       if (!task_state_match(p, match_state))
                                return 0;
                        cpu_relax();
                }
@@ -2272,8 +2305,15 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
                running = task_on_cpu(rq, p);
                queued = task_on_rq_queued(p);
                ncsw = 0;
-               if (READ_ONCE(p->__state) & match_state)
+               if ((match = __task_state_match(p, match_state))) {
+                       /*
+                        * When matching on p->saved_state, consider this task
+                        * still queued so it will wait.
+                        */
+                       if (match < 0)
+                               queued = 1;
                        ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
+               }
                task_rq_unlock(rq, p, &rf);
 
                /*
@@ -4003,15 +4043,14 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
 static __always_inline
 bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
 {
+       int match;
+
        if (IS_ENABLED(CONFIG_DEBUG_PREEMPT)) {
                WARN_ON_ONCE((state & TASK_RTLOCK_WAIT) &&
                             state != TASK_RTLOCK_WAIT);
        }
 
-       if (READ_ONCE(p->__state) & state) {
-               *success = 1;
-               return true;
-       }
+       *success = !!(match = __task_state_match(p, state));
 
 #ifdef CONFIG_PREEMPT_RT
        /*
@@ -4027,12 +4066,10 @@ bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
         * p::saved_state to TASK_RUNNING so any further tests will
         * not result in false positives vs. @success
         */
-       if (p->saved_state & state) {
+       if (match < 0)
                p->saved_state = TASK_RUNNING;
-               *success = 1;
-       }
 #endif
-       return false;
+       return match > 0;
 }
 
 /*