Re: [Qemu-devel] [PATCH v2] 9pfs: Correctly handle cancelled requests

[Stefano Stabellini]

On Wed, 20 Dec 2017, Greg Kurz wrote:
> On Tue, 12 Dec 2017 15:14:06 -0800 (PST)
> Stefano Stabellini <address@hidden> wrote:
> 
> > On Fri, 8 Dec 2017, Greg Kurz wrote:
> > > Cc'ing Stefano using a more appropriate address :)  
> > 
> > Thanks Greg for the ping, one comment inline below.
> > 
> > 
> > > On Thu, 7 Dec 2017 18:04:24 +0100
> > > Greg Kurz <address@hidden> wrote:
> > >   
> > > > On Mon, 4 Dec 2017 15:36:19 -0500
> > > > Keno Fischer <address@hidden> wrote:
> > > >   
> > > > >  # Background
> > > > > 
> > > > > I was investigating spurious non-deterministic EINTR returns from
> > > > > various 9p file system operations in a Linux guest served from the
> > > > > qemu 9p server.
> > > > > 
> > > > >  ## EINTR, ERESTARTSYS and the linux kernel
> > > > > 
> > > > > When a signal arrives that the Linux kernel needs to deliver to 
> > > > > user-space
> > > > > while a given thread is blocked (in the 9p case waiting for a reply 
> > > > > to its
> > > > > request in 9p_client_rpc -> wait_event_interruptible), it asks 
> > > > > whatever
> > > > > driver is currently running to abort its current operation (in the 9p 
> > > > > case
> > > > > causing the submission of a TFLUSH message) and return to user space.
> > > > > In these situations, the error message reported is generally 
> > > > > ERESTARTSYS.
> > > > > If the userspace processes specified SA_RESTART, this means that the
> > > > > system call will get restarted upon completion of the signal handler
> > > > > delivery (assuming the signal handler doesn't modify the process state
> > > > > in complicated ways not relevant here). If SA_RESTART is not 
> > > > > specified,
> > > > > ERESTARTSYS gets translated to EINTR and user space is expected to 
> > > > > handle
> > > > > the restart itself.
> > > > > 
> > > > >  ## The 9p TFLISH command    
> > > >                ^^^
> > > > I'll fix this before pushing to my tree in case there's no v3.
> > > >   
> > > > > 
> > > > > The 9p TFLUSH commands requests that the server abort an ongoing 
> > > > > operation.
> > > > > The man page [1] specifies:
> > > > > 
> > > > > ```
> > > > > If it recognizes oldtag as the tag of a pending transaction, it 
> > > > > should abort any
> > > > > pending response and discard that tag.
> > > > > [...]
> > > > > When the client sends a Tflush, it must wait to receive the 
> > > > > corresponding Rflush
> > > > > before reusing oldtag for subsequent messages. If a response to the 
> > > > > flushed request
> > > > > is received before the Rflush, the client must honor the response as 
> > > > > if it had not
> > > > > been flushed, since the completed request may signify a state change 
> > > > > in the server
> > > > > ```
> > > > > 
> > > > > In particular, this means that the server must not send a reply with 
> > > > > the orignal
> > > > > tag in response to the cancellation request, because the client is 
> > > > > obligated
> > > > > to interpret such a reply as a coincidental reply to the original 
> > > > > request.
> > > > > 
> > > > >  # The bug
> > > > > 
> > > > > When qemu receives a TFlush request, it sets the `cancelled` flag on 
> > > > > the relevant
> > > > > pdu. This flag is periodically checked, e.g. in 
> > > > > `v9fs_co_name_to_path`, and if
> > > > > set, the operation is aborted and the error is set to EINTR. However, 
> > > > > the server
> > > > > then violates the spec, by returning to the client an Rerror 
> > > > > response, rather
> > > > > than discarding the message entirely. As a result, the client is 
> > > > > required
> > > > > to assume that said Rerror response is a result of the original 
> > > > > request, not
> > > > > a result of the cancellation and thus passes the EINTR error back to 
> > > > > user space.
> > > > > This is not the worst thing it could do, however as discussed above, 
> > > > > the correct
> > > > > error code would have been ERESTARTSYS, such that user space programs 
> > > > > with
> > > > > SA_RESTART set get correctly restarted upon completion of the signal 
> > > > > handler.
> > > > > Instead, such programs get spurious EINTR results that they were not 
> > > > > expecting
> > > > > to handle.
> > > > > 
> > > > > It should be noted that there are plenty of user space programs that 
> > > > > do not
> > > > > set SA_RESTART and do not correctly handle EINTR either. However, 
> > > > > that is then
> > > > > a userspace bug. It should also be noted that this bug has been 
> > > > > mitigated by
> > > > > a recent commit to the Linux kernel [2], which essentially prevents 
> > > > > the kernel
> > > > > from sending Tflush requests unless the process is about to die (in 
> > > > > which case
> > > > > the process likely doesn't care about the response). Nevertheless, 
> > > > > for older
> > > > > kernels and to comply with the spec, I believe this change is 
> > > > > beneficial.
> > > > > 
> > > > >  # Implementation
> > > > > 
> > > > > The fix is fairly simple, just skipping notification of a reply if
> > > > > the pdu was previously cancelled. We do however, also notify the 
> > > > > transport
> > > > > layer that we're doing this, so it can clean up any resources it may 
> > > > > be
> > > > > holding. I also added a new trace event to distinguish
> > > > > operations that caused an error reply from those that were cancelled.
> > > > > 
> > > > > One complication is that we only omit sending the message on EINTR 
> > > > > errors in
> > > > > order to avoid confusing the rest of the code (which may assume that a
> > > > > client knows about a fid if it sucessfully passed it off to 
> > > > > pud_complete
> > > > > without checking for cancellation status). This does mean that if the 
> > > > > server
> > > > > acts upon the cancellation flag, it always needs to set err to EINTR. 
> > > > > I believe
> > > > > this is true of the current code.
> > > > > 
> > > > > [1] https://9fans.github.io/plan9port/man/man9/flush.html
> > > > > [2] 
> > > > > https://github.com/torvalds/linux/commit/9523feac272ccad2ad8186ba4fcc89103754de52
> > > > > 
> > > > > Signed-off-by: Keno Fischer <address@hidden>
> > > > > ---
> > > > >     
> > > > 
> > > > It looks good. I could come up with a new qtest for TFLUSH and validate 
> > > > it
> > > > behaves as expected. I'll post that soon.
> > > > 
> > > > For the 9p and virtio parts.
> > > > 
> > > > Reviewed-by: Greg Kurz <address@hidden>
> > > > 
> > > > Now I'd need an ack from Stefano for the xen bits before I push it to 
> > > > my tree.
> > > > 
> > > > Thanks,
> > > > 
> > > > --
> > > > Greg
> > > >   
> > > > > Changes from v1:
> > > > >  - In reponse to review by Greg Kurz, add a new transport
> > > > >    layer operation to discard the buffer. I also attempted
> > > > >    an implementation for xen, but I have done no verification
> > > > >    on that beyond making sure it compiles, since I don't know
> > > > >    how to use xen. Please review closely.
> > > > > 
> > > > >  hw/9pfs/9p.c               | 18 ++++++++++++++++++
> > > > >  hw/9pfs/9p.h               |  1 +
> > > > >  hw/9pfs/trace-events       |  1 +
> > > > >  hw/9pfs/virtio-9p-device.c | 14 ++++++++++++++
> > > > >  hw/9pfs/xen-9p-backend.c   | 11 +++++++++++
> > > > >  5 files changed, 45 insertions(+)
> > > > > 
> > > > > diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> > > > > index 710cd91..daa8519 100644
> > > > > --- a/hw/9pfs/9p.c
> > > > > +++ b/hw/9pfs/9p.c
> > > > > @@ -648,6 +648,23 @@ static void coroutine_fn pdu_complete(V9fsPDU 
> > > > > *pdu, ssize_t len)
> > > > >      V9fsState *s = pdu->s;
> > > > >      int ret;
> > > > >  
> > > > > +    /*
> > > > > +     * The 9p spec requires that successfully cancelled pdus receive 
> > > > > no reply.
> > > > > +     * Sending a reply would confuse clients because they would
> > > > > +     * assume that any EINTR is the actual result of the operation,
> > > > > +     * rather than a consequence of the cancellation. However, if
> > > > > +     * the operation completed (succesfully or with an error other
> > > > > +     * than caused be cancellation), we do send out that reply, both
> > > > > +     * for efficiency and to avoid confusing the rest of the state 
> > > > > machine
> > > > > +     * that assumes passing a non-error here will mean a successful
> > > > > +     * transmission of the reply.
> > > > > +     */
> > > > > +    if (pdu->cancelled && len == -EINTR) {
> > > > > +        trace_v9fs_rcancel(pdu->tag, pdu->id);
> > > > > +        pdu->s->transport->discard(pdu);
> > > > > +        goto out_wakeup;
> > > > > +    }
> > > > > +
> > > > >      if (len < 0) {
> > > > >          int err = -len;
> > > > >          len = 7;
> > > > > @@ -690,6 +707,7 @@ static void coroutine_fn pdu_complete(V9fsPDU 
> > > > > *pdu, ssize_t len)
> > > > >  out_notify:
> > > > >      pdu->s->transport->push_and_notify(pdu);
> > > > >  
> > > > > +out_wakeup:
> > > > >      /* Now wakeup anybody waiting in flush for this request */
> > > > >      if (!qemu_co_queue_next(&pdu->complete)) {
> > > > >          pdu_free(pdu);
> > > > > diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
> > > > > index d1cfeaf..3c1b0b5 100644
> > > > > --- a/hw/9pfs/9p.h
> > > > > +++ b/hw/9pfs/9p.h
> > > > > @@ -365,6 +365,7 @@ struct V9fsTransport {
> > > > >      void        (*init_out_iov_from_pdu)(V9fsPDU *pdu, struct iovec 
> > > > > **piov,
> > > > >                                           unsigned int *pniov, size_t 
> > > > > size);
> > > > >      void        (*push_and_notify)(V9fsPDU *pdu);
> > > > > +    void        (*discard)(V9fsPDU *pdu);
> > > > >  };
> > > > >  
> > > > >  static inline int v9fs_register_transport(V9fsState *s,
> > > > > diff --git a/hw/9pfs/trace-events b/hw/9pfs/trace-events
> > > > > index 08a4abf..1aee350 100644
> > > > > --- a/hw/9pfs/trace-events
> > > > > +++ b/hw/9pfs/trace-events
> > > > > @@ -1,6 +1,7 @@
> > > > >  # See docs/devel/tracing.txt for syntax documentation.
> > > > >  
> > > > >  # hw/9pfs/virtio-9p.c
> > > > > +v9fs_rcancel(uint16_t tag, uint8_t id) "tag %d id %d"
> > > > >  v9fs_rerror(uint16_t tag, uint8_t id, int err) "tag %d id %d err %d"
> > > > >  v9fs_version(uint16_t tag, uint8_t id, int32_t msize, char* version) 
> > > > > "tag %d id %d msize %d version %s"
> > > > >  v9fs_version_return(uint16_t tag, uint8_t id, int32_t msize, char* 
> > > > > version) "tag %d id %d msize %d version %s"
> > > > > diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
> > > > > index 62650b0..2510329 100644
> > > > > --- a/hw/9pfs/virtio-9p-device.c
> > > > > +++ b/hw/9pfs/virtio-9p-device.c
> > > > > @@ -37,6 +37,19 @@ static void virtio_9p_push_and_notify(V9fsPDU *pdu)
> > > > >      virtio_notify(VIRTIO_DEVICE(v), v->vq);
> > > > >  }
> > > > >  
> > > > > +
> > > > > +static void virtio_pdu_discard(V9fsPDU *pdu)
> > > > > +{
> > > > > +    V9fsState *s = pdu->s;
> > > > > +    V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
> > > > > +    VirtQueueElement *elem = v->elems[pdu->idx];
> > > > > +
> > > > > +    /* discard element from the queue */
> > > > > +    virtqueue_detach_element(v->vq, elem, pdu->size);
> > > > > +    g_free(elem);
> > > > > +    v->elems[pdu->idx] = NULL;
> > > > > +}
> > > > > +
> > > > >  static void handle_9p_output(VirtIODevice *vdev, VirtQueue *vq)
> > > > >  {
> > > > >      V9fsVirtioState *v = (V9fsVirtioState *)vdev;
> > > > > @@ -221,6 +234,7 @@ static const struct V9fsTransport 
> > > > > virtio_9p_transport = {
> > > > >      .init_in_iov_from_pdu = virtio_init_in_iov_from_pdu,
> > > > >      .init_out_iov_from_pdu = virtio_init_out_iov_from_pdu,
> > > > >      .push_and_notify = virtio_9p_push_and_notify,
> > > > > +    .discard = virtio_pdu_discard,
> > > > >  };
> > > > >  
> > > > >  /* virtio-9p device */
> > > > > diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
> > > > > index ee87f08..7208ce6 100644
> > > > > --- a/hw/9pfs/xen-9p-backend.c
> > > > > +++ b/hw/9pfs/xen-9p-backend.c
> > > > > @@ -233,12 +233,23 @@ static void xen_9pfs_push_and_notify(V9fsPDU 
> > > > > *pdu)
> > > > >      qemu_bh_schedule(ring->bh);
> > > > >  }
> > > > >  
> > > > > +static void xen_9pfs_discard(V9fsPDU *pdu)
> > > > > +{
> > > > > +    Xen9pfsDev *priv = container_of(pdu->s, Xen9pfsDev, state);
> > > > > +    Xen9pfsRing *ring = &priv->rings[pdu->tag % priv->num_rings];
> > > > > +
> > > > > +    g_free(ring->sg);
> > > > > +    ring->sg = NULL;
> > > > > +    ring->inprogress = false;  
> > 
> > I think we also need to:
> > 
> >     /* this is required to consume the request */
> >     ring->intf->out_cons = ring->out_cons;
> >     xen_wmb();
> > 
> >     /* this is required to complete the cancellation of the pdu operation */
> >     ring->inprogress = false;
> > 
> >     /* this is required to let the other end know that there is more
> >      * space on the ring available */
> >     xenevtchn_notify(ring->evtchndev, ring->local_port);
> > 
> 
> Actually I'm not sure you should notify the other end. The 9p client in linux
> already has a 'cancelled' transport callback for this exact purpose. It is
> currently implemented by the rdma and fd transports only, but I have a working
> prototype with virtio I expect to send soon. I guess xen should go that way
> too.

Even if we don't write anything back, we would still need to notify the
other end because the request was read, meaning that there is more space
available on the out ring (the ring indexes have been increased). It is
basically part of the underlying byte tranfer protocol, indepent from
the meaning of those bytes.


> >     /* trigger the read of the next request */
> >     qemu_bh_schedule(ring->bh);
> > 
> > 
> > Which is basically almost everything else from xen_9pfs_push_and_notify,
> > except the modification of in_prod, because we are not going to write
> > back anything to the ring.
> > 
> > 
> > 
> > > > > +}
> > > > > +
> > > > >  static const struct V9fsTransport xen_9p_transport = {
> > > > >      .pdu_vmarshal = xen_9pfs_pdu_vmarshal,
> > > > >      .pdu_vunmarshal = xen_9pfs_pdu_vunmarshal,
> > > > >      .init_in_iov_from_pdu = xen_9pfs_init_in_iov_from_pdu,
> > > > >      .init_out_iov_from_pdu = xen_9pfs_init_out_iov_from_pdu,
> > > > >      .push_and_notify = xen_9pfs_push_and_notify,
> > > > > +    .discard = xen_9pfs_discard,
> > > > >  };
> > > > >  
> > > > >  static int xen_9pfs_init(struct XenDevice *xendev)    
> > 
>