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

[Kevin Wolf <kwolf@redhat.com>]

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Am 04.12.2023 um 17:30 hat Stefan Hajnoczi geschrieben:
> 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().

Hm, I wonder if that is actually a bug. Without executing pending BHs,
you can't be sure that nothing touches the node any more.

Before commit 5e8ac217, we always polled at least once, though I think
that was an unintentional side effect.

> > 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.

Why is it correct when there are no requests? I can see this for
scsi_device_purge_requests() because it only cancels in-flight requests,
but scsi_dma_restart_cb() is about requests queued at the device level
that are not in flight in the block layer. Not restarting them if there
aren't any other requests in flight seems wrong.

> > > +
> > > +    /* 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.

Ok, then the answer for the above is less important.

Kevin

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