Re: [Virtio-fs] [PATCH 4/4] virtiofsd: Implement blocking posix locks

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Vivek Goyal (vgoyal@redhat.com) wrote:
> On Tue, Nov 26, 2019 at 01:02:29PM +0000, Dr. David Alan Gilbert wrote:
> 
> [..]
> > > > > @@ -1950,21 +1948,54 @@ static void lo_setlk(fuse_req_t req, fuse_ino_t ino,
> > > > >  
> > > > >  	if (!plock) {
> > > > >  		saverr = ret;
> > > > > +		pthread_mutex_unlock(&inode->plock_mutex);
> > > > >  		goto out;
> > > > >  	}
> > > > >  
> > > > > +	/*
> > > > > +	 * plock is now released when inode is going away. We already have
> > > > > +	 * a reference on inode, so it is guaranteed that plock->fd is
> > > > > +	 * still around even after dropping inode->plock_mutex lock
> > > > > +	 */
> > > > > +	ofd = plock->fd;
> > > > > +	pthread_mutex_unlock(&inode->plock_mutex);
> > > > > +
> > > > > +	/*
> > > > > +	 * If this lock request can block, request caller to wait for
> > > > > +	 * notification. Do not access req after this. Once lock is
> > > > > +	 * available, send a notification instead.
> > > > > +	 */
> > > > > +	if (sleep && lock->l_type != F_UNLCK) {
> > > > > +		/*
> > > > > +		 * If notification queue is not enabled, can't support async
> > > > > +		 * locks.
> > > > > +		 */
> > > > > +		if (!se->notify_enabled) {
> > > > > +			saverr = EOPNOTSUPP;
> > > > > +			goto out;
> > > > > +		}
> > > > > +		async_lock = true;
> > > > > +		unique = req->unique;
> > > > > +		fuse_reply_wait(req);
> > > > > +	}
> > > > >  	/* TODO: Is it alright to modify flock? */
> > > > >  	lock->l_pid = 0;
> > > > > -	ret = fcntl(plock->fd, F_OFD_SETLK, lock);
> > > > > +	if (async_lock)
> > > > > +		ret = fcntl(ofd, F_OFD_SETLKW, lock);
> > > > > +	else
> > > > > +		ret = fcntl(ofd, F_OFD_SETLK, lock);
> > > > 
> > > > What happens if the guest is rebooted after it's asked
> > > > for, but not been granted a lock?
> > > 
> > > I think a regular reboot can't be done till a request is pending, because
> > > virtio-fs can't be unmounted and unmount will wait for all pending
> > > requests to finish.
> > > 
> > > Destroying qemu will destroy deamon too.
> > > 
> > > Are there any other reboot paths I have missed.
> > 
> > Yes, there are a few other ways the guest can reboot:
> >   a) A echo b > /proc/sysrq-trigger
> 
> I tried it. Both qemu and virtiofsd hang. virtiofsd wants to stop a 
> queue. And that tries to stop thrad pool. But one of the threads in
> thread pool is blocked on setlkw. So g_thread_pool_free() hangs.
> 
> I am not seeing any option in glib thread pool API to stop or send
> signal to threads which are blocked.

Is there a way to setup pthread_cancel ?  The upstream libfuse code
has somec ases where it enables cancellation very carefully around
something that might block, does it, then disables cancellation.

Dave

> Thanks
> Vivek
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK