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 <gr...@kaod.org> wrote: > > > On Mon, 4 Dec 2017 15:36:19 -0500 > > Keno Fischer <k...@juliacomputing.com> 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 <k...@juliacomputing.com> > > > --- > > > > > > > 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 <gr...@kaod.org> > > > > 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); /* 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)
