Re: [PATCH 1/4] scsi: only access SCSIDevice->requests from one thread

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Dec 01, 2023 at 05:03:13PM +0100, Kevin Wolf wrote:
> Am 23.11.2023 um 20:49 hat Stefan Hajnoczi geschrieben:
> > Stop depending on the AioContext lock and instead access
> > SCSIDevice->requests from only one thread at a time:
> > - When the VM is running only the BlockBackend's AioContext may access
> >   the requests list.
> > - When the VM is stopped only the main loop may access the requests
> >   list.
> > 
> > These constraints protect the requests list without the need for locking
> > in the I/O code path.
> > 
> > Note that multiple IOThreads are not supported yet because the code
> > assumes all SCSIRequests are executed from a single AioContext. Leave
> > that as future work.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  include/hw/scsi/scsi.h |   7 +-
> >  hw/scsi/scsi-bus.c     | 174 ++++++++++++++++++++++++++++-------------
> >  2 files changed, 124 insertions(+), 57 deletions(-)
> > 
> > diff --git a/include/hw/scsi/scsi.h b/include/hw/scsi/scsi.h
> > index 3692ca82f3..10c4e8288d 100644
> > --- a/include/hw/scsi/scsi.h
> > +++ b/include/hw/scsi/scsi.h
> > @@ -69,14 +69,19 @@ struct SCSIDevice
> >  {
> >      DeviceState qdev;
> >      VMChangeStateEntry *vmsentry;
> > -    QEMUBH *bh;
> >      uint32_t id;
> >      BlockConf conf;
> >      SCSISense unit_attention;
> >      bool sense_is_ua;
> >      uint8_t sense[SCSI_SENSE_BUF_SIZE];
> >      uint32_t sense_len;
> > +
> > +    /*
> > +     * The requests list is only accessed from the AioContext that executes
> > +     * requests or from the main loop when IOThread processing is stopped.
> > +     */
> >      QTAILQ_HEAD(, SCSIRequest) requests;
> > +
> >      uint32_t channel;
> >      uint32_t lun;
> >      int blocksize;
> > diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
> > index fc4b77fdb0..b8bfde9565 100644
> > --- a/hw/scsi/scsi-bus.c
> > +++ b/hw/scsi/scsi-bus.c
> > @@ -85,6 +85,82 @@ SCSIDevice *scsi_device_get(SCSIBus *bus, int channel, int id, int lun)
> >      return d;
> >  }
> >  
> > +/*
> > + * Invoke @fn() for each enqueued request in device @s. Must be called from the
> > + * main loop thread while the guest is stopped. This is only suitable for
> > + * vmstate ->put(), use scsi_device_for_each_req_async() for other cases.
> > + */
> > +static void scsi_device_for_each_req_sync(SCSIDevice *s,
> > +                                          void (*fn)(SCSIRequest *, void *),
> > +                                          void *opaque)
> > +{
> > +    SCSIRequest *req;
> > +    SCSIRequest *next_req;
> > +
> > +    assert(!runstate_is_running());
> > +    assert(qemu_in_main_thread());
> > +
> > +    QTAILQ_FOREACH_SAFE(req, &s->requests, next, next_req) {
> > +        fn(req, opaque);
> > +    }
> > +}
> > +
> > +typedef struct {
> > +    SCSIDevice *s;
> > +    void (*fn)(SCSIRequest *, void *);
> > +    void *fn_opaque;
> > +} SCSIDeviceForEachReqAsyncData;
> > +
> > +static void scsi_device_for_each_req_async_bh(void *opaque)
> > +{
> > +    g_autofree SCSIDeviceForEachReqAsyncData *data = opaque;
> > +    SCSIDevice *s = data->s;
> > +    SCSIRequest *req;
> > +    SCSIRequest *next;
> > +
> > +    /*
> > +     * It is unlikely that the AioContext will change before this BH is called,
> > +     * but if it happens then ->requests must not be accessed from this
> > +     * AioContext.
> > +     */
> 
> What is the scenario where this happens? I would have expected that
> switching the AioContext of a node involves draining the node first,
> which would execute this BH before the context changes.

I don't think aio_poll() is invoked by bdrv_drained_begin() when there
are no requests in flight. In that case the BH could remain pending
across bdrv_drained_begin()/bdrv_drained_end().

> The other option I see is an empty BlockBackend, which can change its
> AioContext without polling BHs, but in that case there is no connection
> to other users, so the only change could come from virtio-scsi itself.
> If there is such a case, it would probably be helpful to be specific in
> the comment.
>
> > +    if (blk_get_aio_context(s->conf.blk) == qemu_get_current_aio_context()) {
> > +        QTAILQ_FOREACH_SAFE(req, &s->requests, next, next) {
> > +            data->fn(req, data->fn_opaque);
> > +        }
> > +    }
> 
> Of course, if the situation does happen, the question is why just doing
> nothing is correct. Wouldn't that mean that the guest still sees stuck
> requests?
> 
> Would rescheduling the BH in the new context be better?

In the case where there are no requests it is correct to do nothing, but
it's not a general solution.

> > +
> > +    /* Drop the reference taken by scsi_device_for_each_req_async() */
> > +    object_unref(OBJECT(s));
> > +}
> > +
> > +/*
> > + * Schedule @fn() to be invoked for each enqueued request in device @s. @fn()
> > + * runs in the AioContext that is executing the request.
> > + */
> > +static void scsi_device_for_each_req_async(SCSIDevice *s,
> > +                                           void (*fn)(SCSIRequest *, void *),
> > +                                           void *opaque)
> 
> If we keep the behaviour above (doesn't do anything if the AioContext
> changes), then I think it needs to be documented for this function and
> callers should be explicit about why it's okay.

I think your suggestion to reschedule in the new AioContext is best.

Stefan

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