[Devel] [PATCH] kernel: call task_work_run() before exit_task_namespaces()

[Konstantin Khorenko]

Kirill, please review the patch.

--
Best regards,

Konstantin Khorenko,
Virtuozzo Linux Kernel Team

On 08/07/2017 08:15 PM, Andrei Vagin wrote:
> From: Andrei Vagin <avagin at virtuozzo.com>
>
> This patch solves a problem for a following case. We have a container (a
> group of processes in pid and mount namespaces) with a fuse mount. An
> init process exits and the kernel kills all process in its pid
> namespace. There is a fuse daemon, which handle the fuse mount.
> Currently the kernel kills this process and closes all its file
> descriptors, but __fput() for them is postponed and they will be
> called from a task_work.  Then the kernel starts destroying the mount
> namespace and the fuse mount, it sees that a control descriptor for
> this mount is alive and sends a request to a fuse daemon:
>
> $ cat /proc/4353/task/4355/stack
> [<ffffffffa04c3451>] request_wait_answer+0x91/0x270 [fuse]
> [<ffffffffa04c36b7>] __fuse_request_send+0x87/0xe0 [fuse]
> [<ffffffffa04c6c47>] fuse_request_check_and_send+0x27/0x30 [fuse]
> [<ffffffffa04c6c60>] fuse_request_send+0x10/0x20 [fuse]
> [<ffffffffa04d2f35>] fuse_put_super+0x55/0xc0 [fuse]
> [<ffffffff81218b32>] generic_shutdown_super+0x72/0xf0
> [<ffffffff81218f12>] kill_anon_super+0x12/0x20
> [<ffffffffa04d2577>] fuse_kill_sb_anon+0x47/0x50 [fuse]
> [<ffffffff812194a9>] deactivate_locked_super+0x49/0x80
> [<ffffffff81219526>] deactivate_super+0x46/0x60
> [<ffffffff81237145>] mntput_no_expire+0xc5/0x120
> [<ffffffff812371c4>] mntput+0x24/0x40
> [<ffffffff812372f8>] namespace_unlock+0x118/0x130
> [<ffffffff81239f2b>] put_mnt_ns+0x4b/0x60
> [<ffffffff810b786b>] free_nsproxy+0x1b/0x90
> [<ffffffff810b7a0a>] switch_task_namespaces+0x5a/0x70
> [<ffffffff810b7ae0>] exit_task_namespaces+0x10/0x20
> [<ffffffff8108c883>] do_exit+0x2f3/0xb20
> [<ffffffff8108d12f>] do_group_exit+0x3f/0xa0
> [<ffffffff8109e760>] get_signal_to_deliver+0x1d0/0x6d0
> [<ffffffff8102a357>] do_signal+0x57/0x6b0
> [<ffffffff8102aa0f>] do_notify_resume+0x5f/0xb0
> [<ffffffff8169273d>] int_signal+0x12/0x17
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> But we know that a fuse daemon is already dead and the control
> descriptor isn't closed completely, because __fput() was postponed.
>
> This patch calls task_work_run() before destroying namespaces to
> complete closing all process files.
>
> In the upstream kernel deactivate_super() is called from a task_work too,
> so there is not this problem. But we can't backport these changes from
> the upstream, because they are too big:
>
> commit 48a066e72d970a3e225a9c18690d570c736fc455
> Author: Al Viro <viro at zeniv.linux.org.uk>
> Date:   Sun Sep 29 22:06:07 2013 -0400
>
>     RCU'd vfsmounts
>
> https://jira.sw.ru/browse/PSBM-68266
> Signed-off-by: Andrei Vagin <avagin at virtuozzo.com>
> Signed-off-by: Andrei Vagin <avagin at openvz.org>
> ---
>  include/linux/task_work.h | 9 +++++++--
>  kernel/exit.c             | 9 +++++++++
>  kernel/task_work.c        | 4 ++--
>  3 files changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/task_work.h b/include/linux/task_work.h
> index ca5a1cf..b3af76d 100644
> --- a/include/linux/task_work.h
> +++ b/include/linux/task_work.h
> @@ -14,11 +14,16 @@ init_task_work(struct callback_head *twork, task_work_func_t func)
>
>  int task_work_add(struct task_struct *task, struct callback_head *twork, bool);
>  struct callback_head *task_work_cancel(struct task_struct *, task_work_func_t);
> -void task_work_run(void);
> +void __task_work_run(bool exiting);
> +
> +static inline void task_work_run(void)
> +{
> +	return __task_work_run(false);
> +}
>
>  static inline void exit_task_work(struct task_struct *task)
>  {
> -	task_work_run();
> +	__task_work_run(true);
>  }
>
>  #endif	/* _LINUX_TASK_WORK_H */
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 3c83db2..ea54a73 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -827,6 +827,15 @@ void do_exit(long code)
>  	exit_fs(tsk);
>  	if (group_dead)
>  		disassociate_ctty(1);
> +
> +	/*
> +	 * task_work_run() has to be called before exit_task_namespaces(),
> +	 * because fuse_abort_conn() is called from __fput(). If it will not
> +	 * be executed, we can hang in request_wait_answer(). We have seen this
> +	 * situation when a process was the last member of a mount namespace
> +	 * and the mount namespace has a vstorage fuse mount.
> +	 */
> +	task_work_run();
>  	exit_task_namespaces(tsk);
>  	exit_task_work(tsk);
>
> diff --git a/kernel/task_work.c b/kernel/task_work.c
> index 65bd3c9..f0000c4 100644
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -46,7 +46,7 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
>  	return work;
>  }
>
> -void task_work_run(void)
> +void __task_work_run(bool exiting)
>  {
>  	struct task_struct *task = current;
>  	struct callback_head *work, *head, *next;
> @@ -58,7 +58,7 @@ void task_work_run(void)
>  		 */
>  		do {
>  			work = ACCESS_ONCE(task->task_works);
> -			head = !work && (task->flags & PF_EXITING) ?
> +			head = !work && exiting ?
>  				&work_exited : NULL;
>  		} while (cmpxchg(&task->task_works, work, head) != work);
>
>