From: Frederic Weisbecker frederic@kernel.org
[ Upstream commit e4e1e8089c5fd948da12cb9f4adc93821036945f ]
Make sure we don't need to look again into the depths of git blame in order not to miss a subtle part about how rcu-tasks is dealing with exiting tasks.
Suggested-by: Boqun Feng boqun.feng@gmail.com Suggested-by: Neeraj Upadhyay quic_neeraju@quicinc.com Suggested-by: Paul E. McKenney paulmck@kernel.org Cc: Oleg Nesterov oleg@redhat.com Cc: Lai Jiangshan jiangshanlai@gmail.com Cc: Eric W. Biederman ebiederm@xmission.com Signed-off-by: Frederic Weisbecker frederic@kernel.org Signed-off-by: Paul E. McKenney paulmck@kernel.org Stable-dep-of: 28319d6dc5e2 ("rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()") Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/rcu/tasks.h | 37 +++++++++++++++++++++++++++++-------- 1 file changed, 29 insertions(+), 8 deletions(-)
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 8b51e6a5b3869..372644126b0de 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -416,11 +416,21 @@ static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop) static void rcu_tasks_postscan(struct list_head *hop) { /* - * Wait for tasks that are in the process of exiting. This - * does only part of the job, ensuring that all tasks that were - * previously exiting reach the point where they have disabled - * preemption, allowing the later synchronize_rcu() to finish - * the job. + * Exiting tasks may escape the tasklist scan. Those are vulnerable + * until their final schedule() with TASK_DEAD state. To cope with + * this, divide the fragile exit path part in two intersecting + * read side critical sections: + * + * 1) An _SRCU_ read side starting before calling exit_notify(), + * which may remove the task from the tasklist, and ending after + * the final preempt_disable() call in do_exit(). + * + * 2) An _RCU_ read side starting with the final preempt_disable() + * call in do_exit() and ending with the final call to schedule() + * with TASK_DEAD state. + * + * This handles the part 1). And postgp will handle part 2) with a + * call to synchronize_rcu(). */ synchronize_srcu(&tasks_rcu_exit_srcu); } @@ -487,7 +497,10 @@ static void rcu_tasks_postgp(struct rcu_tasks *rtp) * * In addition, this synchronize_rcu() waits for exiting tasks * to complete their final preempt_disable() region of execution, - * cleaning up after the synchronize_srcu() above. + * cleaning up after synchronize_srcu(&tasks_rcu_exit_srcu), + * enforcing the whole region before tasklist removal until + * the final schedule() with TASK_DEAD state to be an RCU TASKS + * read side critical section. */ synchronize_rcu(); } @@ -576,7 +589,11 @@ static void show_rcu_tasks_classic_gp_kthread(void) } #endif /* #ifndef CONFIG_TINY_RCU */
-/* Do the srcu_read_lock() for the above synchronize_srcu(). */ +/* + * Contribute to protect against tasklist scan blind spot while the + * task is exiting and may be removed from the tasklist. See + * corresponding synchronize_srcu() for further details. + */ void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu) { preempt_disable(); @@ -584,7 +601,11 @@ void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu) preempt_enable(); }
-/* Do the srcu_read_unlock() for the above synchronize_srcu(). */ +/* + * Contribute to protect against tasklist scan blind spot while the + * task is exiting and may be removed from the tasklist. See + * corresponding synchronize_srcu() for further details. + */ void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu) { struct task_struct *t = current;