Re: strange interaction between fuse + pidns

Vivek Goyal
2022-06-24T17:37:46Z

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
>