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

[Greg Kurz]

Cc'ing Stefano using a more appropriate address :)

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;
> > +}
> > +
> >  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)  
> 
>