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Date:   Fri, 24 Jun 2022 13:36:59 -0400
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
Message-ID: <YrX2O4Yv8elsQkF9@redhat.com>
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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
>