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 <stefa...@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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