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2024-10-12 a5969cabbb4660eab42b6ef0412cbbd1200cf14d 
 kernel/kernel/rcu/tasks.h
....@@ -171,8 +171,9 @@
171171 static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
172172 {
173173 	/* Complain if the scheduler has not started.  */
174
-	WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
175
-			 "synchronize_rcu_tasks called too soon");
174
+	if (WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
175
+			 "synchronize_%s() called too soon", rtp->name))
176
+		return;
176177 
177178 	/* Wait for the grace period. */
178179 	wait_rcu_gp(rtp->call_func);
....@@ -416,11 +417,21 @@
416417 static void rcu_tasks_postscan(struct list_head *hop)
417418 {
418419 	/*
419
-	 * Wait for tasks that are in the process of exiting.  This
420
-	 * does only part of the job, ensuring that all tasks that were
421
-	 * previously exiting reach the point where they have disabled
422
-	 * preemption, allowing the later synchronize_rcu() to finish
423
-	 * the job.
420
+	 * Exiting tasks may escape the tasklist scan. Those are vulnerable
421
+	 * until their final schedule() with TASK_DEAD state. To cope with
422
+	 * this, divide the fragile exit path part in two intersecting
423
+	 * read side critical sections:
424
+	 *
425
+	 * 1) An _SRCU_ read side starting before calling exit_notify(),
426
+	 *    which may remove the task from the tasklist, and ending after
427
+	 *    the final preempt_disable() call in do_exit().
428
+	 *
429
+	 * 2) An _RCU_ read side starting with the final preempt_disable()
430
+	 *    call in do_exit() and ending with the final call to schedule()
431
+	 *    with TASK_DEAD state.
432
+	 *
433
+	 * This handles the part 1). And postgp will handle part 2) with a
434
+	 * call to synchronize_rcu().
424435 	 */
425436 	synchronize_srcu(&tasks_rcu_exit_srcu);
426437 }
....@@ -487,7 +498,10 @@
487498 	 *
488499 	 * In addition, this synchronize_rcu() waits for exiting tasks
489500 	 * to complete their final preempt_disable() region of execution,
490
-	 * cleaning up after the synchronize_srcu() above.
501
+	 * cleaning up after synchronize_srcu(&tasks_rcu_exit_srcu),
502
+	 * enforcing the whole region before tasklist removal until
503
+	 * the final schedule() with TASK_DEAD state to be an RCU TASKS
504
+	 * read side critical section.
491505 	 */
492506 	synchronize_rcu();
493507 }
....@@ -576,28 +590,43 @@
576590 }
577591 #endif /* #ifndef CONFIG_TINY_RCU */
578592 
579
-/* Do the srcu_read_lock() for the above synchronize_srcu().  */
593
+/*
594
+ * Contribute to protect against tasklist scan blind spot while the
595
+ * task is exiting and may be removed from the tasklist. See
596
+ * corresponding synchronize_srcu() for further details.
597
+ */
580598 void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
581599 {
582
-	preempt_disable();
583600 	current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
584
-	preempt_enable();
585601 }
586602 
587
-/* Do the srcu_read_unlock() for the above synchronize_srcu().  */
588
-void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
603
+/*
604
+ * Contribute to protect against tasklist scan blind spot while the
605
+ * task is exiting and may be removed from the tasklist. See
606
+ * corresponding synchronize_srcu() for further details.
607
+ */
608
+void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu)
589609 {
590610 	struct task_struct *t = current;
591611 
592
-	preempt_disable();
593612 	__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
594
-	preempt_enable();
595
-	exit_tasks_rcu_finish_trace(t);
613
+}
614
+
615
+/*
616
+ * Contribute to protect against tasklist scan blind spot while the
617
+ * task is exiting and may be removed from the tasklist. See
618
+ * corresponding synchronize_srcu() for further details.
619
+ */
620
+void exit_tasks_rcu_finish(void)
621
+{
622
+	exit_tasks_rcu_stop();
623
+	exit_tasks_rcu_finish_trace(current);
596624 }
597625 
598626 #else /* #ifdef CONFIG_TASKS_RCU */
599627 static inline void show_rcu_tasks_classic_gp_kthread(void) { }
600628 void exit_tasks_rcu_start(void) { }
629
+void exit_tasks_rcu_stop(void) { }
601630 void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
602631 #endif /* #else #ifdef CONFIG_TASKS_RCU */
603632 
....@@ -620,9 +649,6 @@
620649 // Wait for one rude RCU-tasks grace period.
621650 static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
622651 {
623
-	if (num_online_cpus() <= 1)
624
-		return;	// Fastpath for only one CPU.
625
-
626652 	rtp->n_ipis += cpumask_weight(cpu_online_mask);
627653 	schedule_on_each_cpu(rcu_tasks_be_rude);
628654 }
....@@ -707,7 +733,7 @@
707733 #endif /* #ifndef CONFIG_TINY_RCU */
708734 
709735 #else /* #ifdef CONFIG_TASKS_RUDE_RCU */
710
-static void show_rcu_tasks_rude_gp_kthread(void) {}
736
+static inline void show_rcu_tasks_rude_gp_kthread(void) {}
711737 #endif /* #else #ifdef CONFIG_TASKS_RUDE_RCU */
712738 
713739 ////////////////////////////////////////////////////////////////////////
....@@ -775,7 +801,7 @@
775801 /* If we are the last reader, wake up the grace-period kthread. */
776802 void rcu_read_unlock_trace_special(struct task_struct *t, int nesting)
777803 {
778
-	int nq = t->trc_reader_special.b.need_qs;
804
+	int nq = READ_ONCE(t->trc_reader_special.b.need_qs);
779805 
780806 	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) &&
781807 	    t->trc_reader_special.b.need_mb)
....@@ -815,33 +841,25 @@
815841 
816842 	// If the task is no longer running on this CPU, leave.
817843 	if (unlikely(texp != t)) {
818
-		if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end)))
819
-			wake_up(&trc_wait);
820844 		goto reset_ipi; // Already on holdout list, so will check later.
821845 	}
822846 
823847 	// If the task is not in a read-side critical section, and
824848 	// if this is the last reader, awaken the grace-period kthread.
825
-	if (likely(!t->trc_reader_nesting)) {
826
-		if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end)))
827
-			wake_up(&trc_wait);
828
-		// Mark as checked after decrement to avoid false
829
-		// positives on the above WARN_ON_ONCE().
849
+	if (likely(!READ_ONCE(t->trc_reader_nesting))) {
830850 		WRITE_ONCE(t->trc_reader_checked, true);
831851 		goto reset_ipi;
832852 	}
833853 	// If we are racing with an rcu_read_unlock_trace(), try again later.
834
-	if (unlikely(t->trc_reader_nesting < 0)) {
835
-		if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end)))
836
-			wake_up(&trc_wait);
854
+	if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0))
837855 		goto reset_ipi;
838
-	}
839856 	WRITE_ONCE(t->trc_reader_checked, true);
840857 
841858 	// Get here if the task is in a read-side critical section.  Set
842859 	// its state so that it will awaken the grace-period kthread upon
843860 	// exit from that critical section.
844
-	WARN_ON_ONCE(t->trc_reader_special.b.need_qs);
861
+	atomic_inc(&trc_n_readers_need_end); // One more to wait on.
862
+	WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs));
845863 	WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
846864 
847865 reset_ipi:
....@@ -856,7 +874,7 @@
856874 static bool trc_inspect_reader(struct task_struct *t, void *arg)
857875 {
858876 	int cpu = task_cpu(t);
859
-	bool in_qs = false;
877
+	int nesting;
860878 	bool ofl = cpu_is_offline(cpu);
861879 
862880 	if (task_curr(t)) {
....@@ -876,23 +894,24 @@
876894 		n_heavy_reader_updates++;
877895 		if (ofl)
878896 			n_heavy_reader_ofl_updates++;
879
-		in_qs = true;
897
+		nesting = 0;
880898 	} else {
881
-		in_qs = likely(!t->trc_reader_nesting);
899
+		// The task is not running, so C-language access is safe.
900
+		nesting = t->trc_reader_nesting;
882901 	}
883902 
884
-	// Mark as checked so that the grace-period kthread will
885
-	// remove it from the holdout list.
886
-	t->trc_reader_checked = true;
887
-
888
-	if (in_qs)
889
-		return true;  // Already in quiescent state, done!!!
903
+	// If not exiting a read-side critical section, mark as checked
904
+	// so that the grace-period kthread will remove it from the
905
+	// holdout list.
906
+	t->trc_reader_checked = nesting >= 0;
907
+	if (nesting <= 0)
908
+		return !nesting;  // If in QS, done, otherwise try again later.
890909 
891910 	// The task is in a read-side critical section, so set up its
892911 	// state so that it will awaken the grace-period kthread upon exit
893912 	// from that critical section.
894913 	atomic_inc(&trc_n_readers_need_end); // One more to wait on.
895
-	WARN_ON_ONCE(t->trc_reader_special.b.need_qs);
914
+	WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs));
896915 	WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
897916 	return true;
898917 }
....@@ -910,7 +929,7 @@
910929 	// The current task had better be in a quiescent state.
911930 	if (t == current) {
912931 		t->trc_reader_checked = true;
913
-		WARN_ON_ONCE(t->trc_reader_nesting);
932
+		WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
914933 		return;
915934 	}
916935 
....@@ -933,21 +952,17 @@
933952 		if (per_cpu(trc_ipi_to_cpu, cpu) || t->trc_ipi_to_cpu >= 0)
934953 			return;
935954 
936
-		atomic_inc(&trc_n_readers_need_end);
937955 		per_cpu(trc_ipi_to_cpu, cpu) = true;
938956 		t->trc_ipi_to_cpu = cpu;
939957 		rcu_tasks_trace.n_ipis++;
940
-		if (smp_call_function_single(cpu,
941
-					     trc_read_check_handler, t, 0)) {
958
+		if (smp_call_function_single(cpu, trc_read_check_handler, t, 0)) {
942959 			// Just in case there is some other reason for
943960 			// failure than the target CPU being offline.
961
+			WARN_ONCE(1, "%s():  smp_call_function_single() failed for CPU: %d\n",
962
+				  __func__, cpu);
944963 			rcu_tasks_trace.n_ipis_fails++;
945964 			per_cpu(trc_ipi_to_cpu, cpu) = false;
946
-			t->trc_ipi_to_cpu = cpu;
947
-			if (atomic_dec_and_test(&trc_n_readers_need_end)) {
948
-				WARN_ON_ONCE(1);
949
-				wake_up(&trc_wait);
950
-			}
965
+			t->trc_ipi_to_cpu = -1;
951966 		}
952967 	}
953968 }
....@@ -1020,8 +1035,8 @@
10201035 		 ".I"[READ_ONCE(t->trc_ipi_to_cpu) > 0],
10211036 		 ".i"[is_idle_task(t)],
10221037 		 ".N"[cpu > 0 && tick_nohz_full_cpu(cpu)],
1023
-		 t->trc_reader_nesting,
1024
-		 " N"[!!t->trc_reader_special.b.need_qs],
1038
+		 READ_ONCE(t->trc_reader_nesting),
1039
+		 " N"[!!READ_ONCE(t->trc_reader_special.b.need_qs)],
10251040 		 cpu);
10261041 	sched_show_task(t);
10271042 }
....@@ -1068,13 +1083,27 @@
10681083 	}
10691084 }
10701085 
1086
+static void rcu_tasks_trace_empty_fn(void *unused)
1087
+{
1088
+}
1089
+
10711090 /* Wait for grace period to complete and provide ordering. */
10721091 static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
10731092 {
1093
+	int cpu;
10741094 	bool firstreport;
10751095 	struct task_struct *g, *t;
10761096 	LIST_HEAD(holdouts);
10771097 	long ret;
1098
+
1099
+	// Wait for any lingering IPI handlers to complete.  Note that
1100
+	// if a CPU has gone offline or transitioned to userspace in the
1101
+	// meantime, all IPI handlers should have been drained beforehand.
1102
+	// Yes, this assumes that CPUs process IPIs in order.  If that ever
1103
+	// changes, there will need to be a recheck and/or timed wait.
1104
+	for_each_online_cpu(cpu)
1105
+		if (smp_load_acquire(per_cpu_ptr(&trc_ipi_to_cpu, cpu)))
1106
+			smp_call_function_single(cpu, rcu_tasks_trace_empty_fn, NULL, 1);
10781107 
10791108 	// Remove the safety count.
10801109 	smp_mb__before_atomic();  // Order vs. earlier atomics
....@@ -1115,7 +1144,7 @@
11151144 static void exit_tasks_rcu_finish_trace(struct task_struct *t)
11161145 {
11171146 	WRITE_ONCE(t->trc_reader_checked, true);
1118
-	WARN_ON_ONCE(t->trc_reader_nesting);
1147
+	WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
11191148 	WRITE_ONCE(t->trc_reader_nesting, 0);
11201149 	if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)))
11211150 		rcu_read_unlock_trace_special(t, 0);