Commit 592031cc authored Sep 15, 2020 by Paul E. McKenney

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: 4593e772 ("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>

parent ba3a86e4

kernel/rcu/tasks.h

+4 −4

Original line number	Diff line number	Diff line
		@@ -1092,10 +1092,10 @@ 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");

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