[Devel] [PATCH] kernel: call task_work_run() before exit_task_namespaces()
Andrey Vagin
avagin at virtuozzo.com
Wed Jul 19 21:36:54 MSK 2017
On Wed, Jul 19, 2017 at 11:31:28AM -0700, Stanislav Kinsburskiy wrote:
>
>
> 19 июля 2017 г. 9:14 PM пользователь Andrey Vagin <avagin at virtuozzo.com>
> написал:
>
> On Wed, Jul 19, 2017 at 08:04:22PM +0300, Andrey Ryabinin wrote:
> >
> >
> > On 07/19/2017 04:14 AM, Andrei Vagin wrote:
> > > From: Andrei Vagin <avagin at virtuozzo.com>
> > >
> > > 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.
> > >
> >
> > Can we pleas have a changelog that doesn't look like an output of random
> text generator?
>
> Thanks!
>
> > The fact that "fuse_abort_conn() is called from __fput()" doesn't really
> explain why
> > task_work_run() needs to be called before exit_task_namespaces.
> >
>
> Here is another version of my random text generator. It has to be more
> detailed.
>
> 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.
>
>
> Now I have a question. Because rised questions look reasonable.
> This sounds like a generic issue.
> I.e. it's either solved in upstream likewise or otherwise. Or not solved at
> all.
> I.e. what's the status of this issue in linux-next?
>
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.
>
>
> >
> >
> > > 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();
> >
> > Given that this is purely fuse's problem, maybe request_wait_answer()
> could just call task_work_run()?
> >
> > Or maybe we can just call exit_task_work(tsk) before exit_task_namespaces
> (tsk). This seems fine to me,
> > but perhaps I'm missing something.
> >
> > > 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 ?
> >
> > Why we need this change? AFAIU this will allow to add more task_works in
> exit_task_namespaces()
> > before final exit_task_work(). What's the point of this?
> >
> > > &work_exited : NULL;
> > > } while (cmpxchg(&task->task_works, work, head) != work);
> > >
> > >
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