From: Vivek Goyal <vgoyal@redhat.com>
To: Tycho Andersen <tycho@tycho.pizza>
Cc: Eric Biederman <ebiederm@xmission.com>,
Christian Brauner <brauner@kernel.org>,
Miklos Szeredi <miklos@szeredi.hu>,
fuse-devel@lists.sourceforge.net, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: strange interaction between fuse + pidns
Date: Fri, 24 Jun 2022 13:36:59 -0400 [thread overview]
Message-ID: <YrX2O4Yv8elsQkF9@redhat.com> (raw)
In-Reply-To: <YrT6Hdqp36HLK9PJ@netflix>
On Thu, Jun 23, 2022 at 05:41:17PM -0600, Tycho Andersen wrote:
> On Thu, Jun 23, 2022 at 05:55:20PM -0400, Vivek Goyal wrote:
> > So in this case single process is client as well as server. IOW, one
> > thread is fuse server servicing fuse requests and other thread is fuse
> > client accessing fuse filesystem?
>
> Yes. Probably an abuse of the API and something people Should Not Do,
> but as you say the kernel still shouldn't lock up like this.
>
> > > since the thread has a copy of
> > > the fd table with an fd pointing to the same fuse device, the reference
> > > count isn't decremented to zero in fuse_dev_release(), and the task hangs
> > > forever.
> >
> > So why did fuse server thread stop responding to fuse messages. Why
> > did it not complete flush.
>
> In this particular case I think it's because the application crashed
> for unrelated reasons and tried to exit the pidns, hitting this
> problem.
>
> > BTW, unkillable wait happens on ly fc->no_interrupt = 1. And this seems
> > to be set only if server probably some previous interrupt request
> > returned -ENOSYS.
> >
> > fuse_dev_do_write() {
> > else if (oh.error == -ENOSYS)
> > fc->no_interrupt = 1;
> > }
> >
> > So a simple workaround might be for server to implement support for
> > interrupting requests.
>
> Yes, but that is the libfuse default IIUC.
Looking at libfuse code. I understand low level API interface and for
that looks like generic code itself will take care of this (without
needing support from filesystem).
libfuse/lib/fuse_lowlevel.c
do_interrupt().
>
> > Having said that, this does sounds like a problem and probably should
> > be fixed at kernel level.
> >
> > >
> > > diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> > > index 0e537e580dc1..c604dfcaec26 100644
> > > --- a/fs/fuse/dev.c
> > > +++ b/fs/fuse/dev.c
> > > @@ -297,7 +297,6 @@ void fuse_request_end(struct fuse_req *req)
> > > spin_unlock(&fiq->lock);
> > > }
> > > WARN_ON(test_bit(FR_PENDING, &req->flags));
> > > - WARN_ON(test_bit(FR_SENT, &req->flags));
> > > if (test_bit(FR_BACKGROUND, &req->flags)) {
> > > spin_lock(&fc->bg_lock);
> > > clear_bit(FR_BACKGROUND, &req->flags);
> > > @@ -381,30 +380,33 @@ static void request_wait_answer(struct fuse_req *req)
> > > queue_interrupt(req);
> > > }
> > >
> > > - if (!test_bit(FR_FORCE, &req->flags)) {
> > > - /* Only fatal signals may interrupt this */
> > > - err = wait_event_killable(req->waitq,
> > > - test_bit(FR_FINISHED, &req->flags));
> > > - if (!err)
> > > - return;
> > > + /* Only fatal signals may interrupt this */
> > > + err = wait_event_killable(req->waitq,
> > > + test_bit(FR_FINISHED, &req->flags));
> >
> > Trying to do a fatal signal killable wait sounds reasonable. But I am
> > not sure about the history.
> >
> > - Why FORCE requests can't do killable wait.
> > - Why flush needs to have FORCE flag set.
>
> args->force implies a few other things besides this killable wait in
> fuse_simple_request(), most notably:
>
> req = fuse_request_alloc(fm, GFP_KERNEL | __GFP_NOFAIL);
>
> and
>
> __set_bit(FR_WAITING, &req->flags);
FR_WAITING stuff is common between both type of requests. We set it
in fuse_get_req() as well which is called for non-force requests.
So there seem to be only two key difference.
- We allocate request with flag __GFP_NOFAIL for force. So don't
want memory allocation to fail.
- And this special casing of non-killable wait.
Miklos probably will have more thoughts on this.
Thanks
Vivek
>
> seems like it probably can be invoked from some non-user/atomic
> context somehow?
>
> > > + if (!err)
> > > + return;
> > >
> > > - spin_lock(&fiq->lock);
> > > - /* Request is not yet in userspace, bail out */
> > > - if (test_bit(FR_PENDING, &req->flags)) {
> > > - list_del(&req->list);
> > > - spin_unlock(&fiq->lock);
> > > - __fuse_put_request(req);
> > > - req->out.h.error = -EINTR;
> > > - return;
> > > - }
> > > + spin_lock(&fiq->lock);
> > > + /* Request is not yet in userspace, bail out */
> > > + if (test_bit(FR_PENDING, &req->flags)) {
> > > + list_del(&req->list);
> > > spin_unlock(&fiq->lock);
> > > + __fuse_put_request(req);
> > > + req->out.h.error = -EINTR;
> > > + return;
> > > }
> > > + spin_unlock(&fiq->lock);
> > >
> > > /*
> > > - * Either request is already in userspace, or it was forced.
> > > - * Wait it out.
> > > + * Womp womp. We sent a request to userspace and now we're getting
> > > + * killed.
> > > */
> > > - wait_event(req->waitq, test_bit(FR_FINISHED, &req->flags));
> > > + set_bit(FR_INTERRUPTED, &req->flags);
> > > + /* matches barrier in fuse_dev_do_read() */
> > > + smp_mb__after_atomic();
> > > + /* request *must* be FR_SENT here, because we ignored FR_PENDING before */
> > > + WARN_ON(!test_bit(FR_SENT, &req->flags));
> > > + queue_interrupt(req);
> > > }
> > >
> > > static void __fuse_request_send(struct fuse_req *req)
> > >
> > > avaialble as a full patch here:
> > > https://github.com/tych0/linux/commit/81b9ff4c8c1af24f6544945da808dbf69a1293f7
> > >
> > > but now things are even weirder. Tasks are stuck at the killable wait, but with
> > > a SIGKILL pending for the thread group.
> >
> > That's strange. No idea what's going on.
>
> Thanks for taking a look. This is where it falls apart for me. In
> principle the patch seems simple, but this sleeping behavior is beyond
> my understanding.
>
> Tycho
>
next prev parent reply other threads:[~2022-06-24 17:37 UTC|newest]
Thread overview: 61+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-23 17:21 strange interaction between fuse + pidns Tycho Andersen
2022-06-23 21:55 ` Vivek Goyal
2022-06-23 23:41 ` Tycho Andersen
2022-06-24 17:36 ` Vivek Goyal [this message]
2022-07-11 10:35 ` Miklos Szeredi
2022-07-11 13:59 ` Miklos Szeredi
2022-07-11 20:25 ` Tycho Andersen
2022-07-11 21:37 ` Eric W. Biederman
2022-07-11 22:53 ` Tycho Andersen
2022-07-11 23:06 ` Eric W. Biederman
2022-07-12 13:43 ` Tycho Andersen
2022-07-12 14:34 ` Eric W. Biederman
2022-07-12 15:14 ` Tycho Andersen
2022-07-13 17:53 ` [PATCH] sched: __fatal_signal_pending() should also check PF_EXITING Tycho Andersen
2022-07-20 15:03 ` Serge E. Hallyn
2022-07-20 20:58 ` Tycho Andersen
2022-07-21 1:54 ` Serge E. Hallyn
2022-07-27 15:44 ` Tycho Andersen
2022-07-27 16:32 ` Eric W. Biederman
2022-07-27 17:55 ` Tycho Andersen
2022-07-28 18:48 ` Eric W. Biederman
2022-07-27 17:55 ` Oleg Nesterov
2022-07-27 18:18 ` Tycho Andersen
2022-07-27 19:19 ` Oleg Nesterov
2022-07-27 19:40 ` Tycho Andersen
2022-07-28 9:12 ` Oleg Nesterov
2022-07-28 21:20 ` Tycho Andersen
2022-07-29 5:04 ` Eric W. Biederman
2022-07-29 13:50 ` Tycho Andersen
2022-07-29 16:15 ` Eric W. Biederman
2022-07-29 16:48 ` Tycho Andersen
2022-07-29 17:40 ` [RFC][PATCH] fuse: In fuse_flush only wait if someone wants the return code Eric W. Biederman
2022-07-29 20:47 ` Oleg Nesterov
2022-07-30 0:15 ` Al Viro
2022-07-30 5:10 ` [RFC][PATCH v2] " Eric W. Biederman
2022-08-01 15:16 ` Tycho Andersen
2022-08-02 12:50 ` Miklos Szeredi
2022-08-15 13:59 ` Tycho Andersen
2022-08-15 17:55 ` Serge E. Hallyn
2022-09-01 14:06 ` [PATCH] " Tycho Andersen
2022-09-19 15:03 ` Tycho Andersen
2022-09-20 18:02 ` Serge E. Hallyn
2022-09-26 14:17 ` Tycho Andersen
2022-09-27 9:46 ` Miklos Szeredi
2022-09-29 14:05 ` [fuse-devel] " Stef Bon
2022-09-29 16:39 ` [PATCH v2] " Tycho Andersen
2022-09-30 13:35 ` Miklos Szeredi
2022-09-30 14:01 ` Tycho Andersen
2022-09-30 14:41 ` Miklos Szeredi
2022-09-30 16:09 ` Tycho Andersen
2022-10-26 9:01 ` Miklos Szeredi
2022-11-14 16:02 ` [PATCH v3] " Tycho Andersen
2022-11-28 15:00 ` Tycho Andersen
2022-12-08 14:26 ` Miklos Szeredi
2022-12-08 17:49 ` Tycho Andersen
2022-12-19 19:16 ` Tycho Andersen
2023-01-03 14:51 ` Tycho Andersen
2023-01-05 15:15 ` Serge E. Hallyn
2023-01-26 14:12 ` Miklos Szeredi
2022-09-30 19:47 ` [PATCH] " Serge E. Hallyn
2022-09-19 15:46 ` [RFC][PATCH v2] " Eric W. Biederman
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