From: "Paul E. McKenney" paulmck@kernel.org

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@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 | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index e583a2d..fcd9c25 100644 --- a/kernel/rcu/tasks.h +++ b/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();