Re: [Virtio-fs] [PATCH 12/13] virtiofsd: Implement blocking posix locks

[Vivek Goyal]

On Wed, Oct 06, 2021 at 05:34:59PM +0200, Christophe de Dinechin wrote:
> 
> On 2021-09-30 at 11:30 -04, Vivek Goyal <vgoyal@redhat.com> wrote...
> > As of now we don't support fcntl(F_SETLKW) and if we see one, we return
> > -EOPNOTSUPP.
> >
> > Change that by accepting these requests and returning a reply
> > immediately asking caller to wait. Once lock is available, send a
> > notification to the waiter indicating lock is available.
> >
> > In response to lock request, we are returning error value as "1", which
> > signals to client to queue the lock request internally and later client
> > will get a notification which will signal lock is taken (or error). And
> > then fuse client should wake up the guest process.
> >
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > Signed-off-by: Ioannis Angelakopoulos <iangelak@redhat.com>
> > ---
> >  tools/virtiofsd/fuse_lowlevel.c  | 37 ++++++++++++++++-
> >  tools/virtiofsd/fuse_lowlevel.h  | 26 ++++++++++++
> >  tools/virtiofsd/fuse_virtio.c    | 50 ++++++++++++++++++++---
> >  tools/virtiofsd/passthrough_ll.c | 70 ++++++++++++++++++++++++++++----
> >  4 files changed, 167 insertions(+), 16 deletions(-)
> >
> > diff --git a/tools/virtiofsd/fuse_lowlevel.c 
> > b/tools/virtiofsd/fuse_lowlevel.c
> > index e4679c73ab..2e7f4b786d 100644
> > --- a/tools/virtiofsd/fuse_lowlevel.c
> > +++ b/tools/virtiofsd/fuse_lowlevel.c
> > @@ -179,8 +179,8 @@ int fuse_send_reply_iov_nofree(fuse_req_t req, int 
> > error, struct iovec *iov,
> >          .unique = req->unique,
> >          .error = error,
> >      };
> > -
> > -    if (error <= -1000 || error > 0) {
> > +    /* error = 1 has been used to signal client to wait for notificaiton */
> > +    if (error <= -1000 || error > 1) {
> 
> What about adding a #define for that special value 1?

Will do. Miklos wants that as well.

> 
> (and while we are at it, the -1000 does not look too good either, that could
> be a separate cleanup patch)

Hmm..., that's an unrelated cleanup. May be for some other day.

> 
> >          fuse_log(FUSE_LOG_ERR, "fuse: bad error value: %i\n", error);
> >          out.error = -ERANGE;
> >      }
> > @@ -290,6 +290,11 @@ int fuse_reply_err(fuse_req_t req, int err)
> >      return send_reply(req, -err, NULL, 0);
> >  }
> >
> > +int fuse_reply_wait(fuse_req_t req)
> > +{
> > +    return send_reply(req, 1, NULL, 0);
> 
> ... to be used here too.

Yes. Wil use new define here too.

> 
> > +}
> > +
> >  void fuse_reply_none(fuse_req_t req)
> >  {
> >      fuse_free_req(req);
> > @@ -2165,6 +2170,34 @@ static void do_destroy(fuse_req_t req, fuse_ino_t 
> > nodeid,
> >      send_reply_ok(req, NULL, 0);
> >  }
> >
> > +static int send_notify_iov(struct fuse_session *se, int notify_code,
> > +                           struct iovec *iov, int count)
> > +{
> > +    struct fuse_out_header out;
> > +    if (!se->got_init) {
> > +        return -ENOTCONN;
> > +    }
> > +    out.unique = 0;
> > +    out.error = notify_code;
> > +    iov[0].iov_base = &out;
> > +    iov[0].iov_len = sizeof(struct fuse_out_header);
> > +    return fuse_send_msg(se, NULL, iov, count);
> > +}
> > +
> > +int fuse_lowlevel_notify_lock(struct fuse_session *se, uint64_t unique,
> > +                  int32_t error)
> > +{
> > +    struct fuse_notify_lock_out outarg = {0};
> > +    struct iovec iov[2];
> > +
> > +    outarg.unique = unique;
> > +    outarg.error = -error;
> > +
> > +    iov[1].iov_base = &outarg;
> > +    iov[1].iov_len = sizeof(outarg);
> > +    return send_notify_iov(se, FUSE_NOTIFY_LOCK, iov, 2);
> > +}
> 
> This may be just me, but I find it odd that you fill iov[0] and iov[1] in
> two separate functions, one of them being static and AFAICT only used once.
> I understand that you are trying to split the notify logic from the lock.
> But the logic is not fully isolated, e.g. the caller needs to know to add
> one to the count, start filling at 1, etc.
> 
> Just a matter of taste, I guess ;-)

I thought that multiple notification types can use common code 
send_notify_iov() because it requires filling common fuse_out_header.
So if in future I introduce another notification say, FUSE_NOTIFY_FOO,
then I can just define one function fuse_lowlevel_notify_foo() and
it can also use send_notify_iov().  I think that's the thought I 
had in mind.


> 
> > +
> >  int fuse_lowlevel_notify_store(struct fuse_session *se, fuse_ino_t ino,
> >                                 off_t offset, struct fuse_bufvec *bufv)
> >  {
> > diff --git a/tools/virtiofsd/fuse_lowlevel.h 
> > b/tools/virtiofsd/fuse_lowlevel.h
> > index c55c0ca2fc..64624b48dc 100644
> > --- a/tools/virtiofsd/fuse_lowlevel.h
> > +++ b/tools/virtiofsd/fuse_lowlevel.h
> > @@ -1251,6 +1251,22 @@ struct fuse_lowlevel_ops {
> >   */
> >  int fuse_reply_err(fuse_req_t req, int err);
> >
> > +/**
> > + * Ask caller to wait for lock.
> > + *
> > + * Possible requests:
> > + *   setlkw
> > + *
> > + * If caller sends a blocking lock request (setlkw), then reply to caller
> > + * that wait for lock to be available. Once lock is available caller will
> > + * receive a notification with request's unique id. Notification will
> > + * carry info whether lock was successfully obtained or not.
> > + *
> > + * @param req request handle
> > + * @return zero for success, -errno for failure to send reply
> > + */
> > +int fuse_reply_wait(fuse_req_t req);
> > +
> >  /**
> >   * Don't send reply
> >   *
> > @@ -1685,6 +1701,16 @@ int fuse_lowlevel_notify_delete(struct fuse_session 
> > *se, fuse_ino_t parent,
> >  int fuse_lowlevel_notify_store(struct fuse_session *se, fuse_ino_t ino,
> >                                 off_t offset, struct fuse_bufvec *bufv);
> >
> > +/**
> > + * Notify event related to previous lock request
> > + *
> > + * @param se the session object
> > + * @param unique the unique id of the request which requested setlkw
> > + * @param error zero for success, -errno for the failure
> > + */
> > +int fuse_lowlevel_notify_lock(struct fuse_session *se, uint64_t unique,
> > +                              int32_t error);
> > +
> >  /*
> >   * Utility functions
> >   */
> > diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c
> > index a87e88e286..bb2d4456fc 100644
> > --- a/tools/virtiofsd/fuse_virtio.c
> > +++ b/tools/virtiofsd/fuse_virtio.c
> > @@ -273,6 +273,23 @@ static void vq_send_element(struct fv_QueueInfo *qi, 
> > VuVirtqElement *elem,
> >      vu_dispatch_unlock(qi->virtio_dev);
> >  }
> >
> > +/* Returns NULL if queue is empty */
> > +static FVRequest *vq_pop_notify_elem(struct fv_QueueInfo *qi)
> > +{
> > +    struct fuse_session *se = qi->virtio_dev->se;
> > +    VuDev *dev = &se->virtio_dev->dev;
> > +    VuVirtq *q = vu_get_queue(dev, qi->qidx);
> > +    FVRequest *req;
> > +
> > +    vu_dispatch_rdlock(qi->virtio_dev);
> > +    pthread_mutex_lock(&qi->vq_lock);
> > +    /* Pop an element from queue */
> > +    req = vu_queue_pop(dev, q, sizeof(FVRequest));
> > +    pthread_mutex_unlock(&qi->vq_lock);
> > +    vu_dispatch_unlock(qi->virtio_dev);
> > +    return req;
> > +}
> > +
> >  /*
> >   * Called back by ll whenever it wants to send a reply/message back
> >   * The 1st element of the iov starts with the fuse_out_header
> > @@ -281,9 +298,9 @@ static void vq_send_element(struct fv_QueueInfo *qi, 
> > VuVirtqElement *elem,
> >  int virtio_send_msg(struct fuse_session *se, struct fuse_chan *ch,
> >                      struct iovec *iov, int count)
> >  {
> > -    FVRequest *req = container_of(ch, FVRequest, ch);
> > -    struct fv_QueueInfo *qi = ch->qi;
> > -    VuVirtqElement *elem = &req->elem;
> > +    FVRequest *req;
> > +    struct fv_QueueInfo *qi;
> > +    VuVirtqElement *elem;
> >      int ret = 0;
> >
> >      assert(count >= 1);
> > @@ -294,8 +311,30 @@ int virtio_send_msg(struct fuse_session *se, struct 
> > fuse_chan *ch,
> >
> >      size_t tosend_len = iov_size(iov, count);
> >
> > -    /* unique == 0 is notification, which we don't support */
> > -    assert(out->unique);
> > +    /* unique == 0 is notification */
> > +    if (!out->unique) {
> > +        if (!se->notify_enabled) {
> > +            return -EOPNOTSUPP;
> > +        }
> > +        /* If notifications are enabled, queue index 1 is notification 
> > queue */
> > +        qi = se->virtio_dev->qi[1];
> > +        req = vq_pop_notify_elem(qi);
> > +        if (!req) {
> > +            /*
> > +             * TODO: Implement some sort of ring buffer and queue 
> > notifications
> > +             * on that and send these later when notification queue has 
> > space
> > +             * available.
> > +             */
> 
> Maybe add a trace / message here to debug more easily if we hit that case?

Maybe I could add a pr_debug() message. But now this code will probably
change. Stefan wants me to wait on some conditional variable for
descriptors to become available (instead of returning -ENOSPC to 
the caller. And be woken up when new descriptors are available (through
queue kick path). In new structure, a message might not be needed.

Thanks
Vivek

> 
> > +            return -ENOSPC;
> > +        }
> > +        req->reply_sent = false;
> > +    } else {
> > +        assert(ch);
> > +        req = container_of(ch, FVRequest, ch);
> > +        qi = ch->qi;
> > +    }
> > +
> > +    elem = &req->elem;
> >      assert(!req->reply_sent);
> >
> >      /* The 'in' part of the elem is to qemu */
> > @@ -985,6 +1024,7 @@ static int fv_get_config(VuDev *dev, uint8_t *config, 
> > uint32_t len)
> >          struct fuse_notify_delete_out       delete_out;
> >          struct fuse_notify_store_out        store_out;
> >          struct fuse_notify_retrieve_out     retrieve_out;
> > +        struct fuse_notify_lock_out         lock_out;
> >      };
> >
> >      notify_size = sizeof(struct fuse_out_header) +
> > diff --git a/tools/virtiofsd/passthrough_ll.c 
> > b/tools/virtiofsd/passthrough_ll.c
> > index 6928662e22..277f74762b 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -2131,13 +2131,35 @@ out:
> >      }
> >  }
> >
> > +static void setlk_send_notification(struct fuse_session *se, uint64_t 
> > unique,
> > +                                    int saverr)
> > +{
> > +    int ret;
> > +
> > +    do {
> > +        ret = fuse_lowlevel_notify_lock(se, unique, saverr);
> > +        /*
> > +         * Retry sending notification if notification queue does not have
> > +         * free descriptor yet, otherwise break out of loop. Either we
> > +         * successfully sent notifiation or some other error occurred.
> > +         */
> > +        if (ret != -ENOSPC) {
> > +            break;
> > +        }
> > +        usleep(10000);
> > +    } while (1);
> > +}
> > +
> >  static void lo_setlk(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info 
> > *fi,
> >                       struct flock *lock, int sleep)
> >  {
> >      struct lo_data *lo = lo_data(req);
> >      struct lo_inode *inode;
> >      struct lo_inode_plock *plock;
> > -    int ret, saverr = 0;
> > +    int ret, saverr = 0, ofd;
> > +    uint64_t unique;
> > +    struct fuse_session *se = req->se;
> > +    bool blocking_lock = false;
> >
> >      fuse_log(FUSE_LOG_DEBUG,
> >               "lo_setlk(ino=%" PRIu64 ", flags=%d)"
> > @@ -2151,11 +2173,6 @@ static void lo_setlk(fuse_req_t req, fuse_ino_t ino, 
> > struct fuse_file_info *fi,
> >          return;
> >      }
> >
> > -    if (sleep) {
> > -        fuse_reply_err(req, EOPNOTSUPP);
> > -        return;
> > -    }
> > -
> >      inode = lo_inode(req, ino);
> >      if (!inode) {
> >          fuse_reply_err(req, EBADF);
> > @@ -2168,21 +2185,56 @@ static void lo_setlk(fuse_req_t req, fuse_ino_t 
> > ino, struct fuse_file_info *fi,
> >
> >      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;
> > +        }
> > +        blocking_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 (blocking_lock) {
> > +        ret = fcntl(ofd, F_OFD_SETLKW, lock);
> > +    } else {
> > +        ret = fcntl(ofd, F_OFD_SETLK, lock);
> > +    }
> >      if (ret == -1) {
> >          saverr = errno;
> >      }
> >
> >  out:
> > -    pthread_mutex_unlock(&inode->plock_mutex);
> >      lo_inode_put(lo, &inode);
> >
> > -    fuse_reply_err(req, saverr);
> > +    if (!blocking_lock) {
> > +        fuse_reply_err(req, saverr);
> > +    } else {
> > +        setlk_send_notification(se, unique, saverr);
> > +    }
> >  }
> >
> >  static void lo_fsyncdir(fuse_req_t req, fuse_ino_t ino, int datasync,
> 
> 
> --
> Cheers,
> Christophe de Dinechin (IRC c3d)
>