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

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Nov 22, 2019 at 05:47:32PM +0000, Dr. David Alan Gilbert wrote:

[..]
> > +static int virtio_send_notify_msg(struct fuse_session *se, struct iovec *iov,
> > +				  int count)
> > +{
> > +    struct fv_QueueInfo *qi;
> > +    VuDev *dev = &se->virtio_dev->dev;
> > +    VuVirtq *q;
> > +    FVRequest *req;
> > +    VuVirtqElement *elem;
> > +    unsigned int in_num, bad_in_num = 0, bad_out_num = 0;
> > +    struct fuse_out_header *out = iov[0].iov_base;
> > +    size_t in_len, tosend_len = iov_size(iov, count);
> > +    struct iovec *in_sg;
> > +    int ret = 0;
> > +
> > +    /* Notifications have unique == 0 */
> > +    assert (!out->unique);
> > +
> > +    if (!se->notify_enabled)
> > +        return -EOPNOTSUPP;
> > +
> > +    /* If notifications are enabled, queue index 1 is notification queue */
> > +    qi = se->virtio_dev->qi[1];
> > +    q = vu_get_queue(dev, qi->qidx);
> > +
> > +    pthread_rwlock_rdlock(&qi->virtio_dev->vu_dispatch_rwlock);
> > +    pthread_mutex_lock(&qi->vq_lock);
> > +    /* Pop an element from queue */
> > +    req = vu_queue_pop(dev, q, sizeof(FVRequest), &bad_in_num, &bad_out_num);
> 
> You don't need bad_in_num/bad_out_num - just pass NULL for both; they're
> only needed if you expect to read/write data that's not mappable (i.e.
> in our direct write case).

Will do.

[..]
> > @@ -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.

Thanks
Vivek