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 <>

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 <>
Date:   Sun Sep 29 22:06:07 2013 -0400

    RCU'd vfsmounts
Signed-off-by: Andrei Vagin <>
Signed-off-by: Andrei Vagin <>
 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)
        if (group_dead)
+       /*
+        * 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();

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 
        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);

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