rcu-tasks: Fix low-probability task_struct leak

When rcu_tasks_trace_postgp() function detects an RCU Tasks Trace
CPU stall, it adds all tasks blocking the current grace period to
a list, invoking get_task_struct() on each to prevent them from
being freed while on the list.  It then traverses that list,
printing stall-warning messages for each one that is still blocking
the current grace period and removing it from the list.  The list
removal invokes the matching put_task_struct().

This of course means that in the admittedly unlikely event that some
task executes its outermost rcu_read_unlock_trace() in the meantime, it
won't be removed from the list and put_task_struct() won't be executing,
resulting in a task_struct leak.  This commit therefore makes the list
removal and put_task_struct() unconditional, stopping the leak.

Note further that this bug can occur only after an RCU Tasks Trace CPU
stall warning, which by default only happens after a grace period has
extended for ten minutes (yes, not a typo, minutes).

Fixes: 4593e772b5 ("rcu-tasks: Add stall warnings for RCU Tasks Trace")
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: <bpf@vger.kernel.org>
Cc: <stable@vger.kernel.org> # 5.7.x
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -1092,11 +1092,11 @@ static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
 			if (READ_ONCE(t->trc_reader_special.b.need_qs))
 				trc_add_holdout(t, &holdouts);
 		firstreport = true;
-		list_for_each_entry_safe(t, g, &holdouts, trc_holdout_list)
-			if (READ_ONCE(t->trc_reader_special.b.need_qs)) {
+		list_for_each_entry_safe(t, g, &holdouts, trc_holdout_list) {
+			if (READ_ONCE(t->trc_reader_special.b.need_qs))
 				show_stalled_task_trace(t, &firstreport);
-				trc_del_holdout(t);
-			}
+			trc_del_holdout(t); // Release task_struct reference.
+		}
 		if (firstreport)
 			pr_err("INFO: rcu_tasks_trace detected stalls? (Counter/taskslist mismatch?)\n");
 		show_stalled_ipi_trace();