linux-kernel - [PATCH 5.9 281/391] rcu-tasks: Fix low-probability task

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Date:   Tue,  3 Nov 2020 21:35:32 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jiri Olsa <jolsa@...hat.com>, bpf@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH 5.9 281/391] rcu-tasks: Fix low-probability task_struct leak

From: Paul E. McKenney <paulmck@...nel.org>

commit 592031cc10858be4adb10f6c0f2608f6f21824aa upstream.

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: 4593e772b502 ("rcu-tasks: Add stall warnings for RCU Tasks Trace")
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Daniel Borkmann <daniel@...earbox.net>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: <bpf@...r.kernel.org>
Cc: <stable@...r.kernel.org> # 5.7.x
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/rcu/tasks.h |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1082,11 +1082,11 @@ static void rcu_tasks_trace_postgp(struc
 			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();