[Devel] [PATCH RHEL7 COMMIT] kernel: call task_work_run() before exit_task_namespaces()

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-514.26.1.vz7.33.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-514.26.1.vz7.33.11
------>
commit 8ec11163ae371bdf08f3e7f5e2420b82ea71c32d
Author: Andrei Vagin <avagin at virtuozzo.com>
Date:   Thu Jul 20 12:41:59 2017 +0400

    kernel: call task_work_run() before exit_task_namespaces()
    
    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, but
    we can't backport these changes from the upstream, because they are too big.
    
    https://jira.sw.ru/browse/PSBM-68266
    
    Signed-off-by: Andrei Vagin <avagin at virtuozzo.com>
---
 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);