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

[Greg Kurz]

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)