On 31/08/22 04:58PM, SeongJae Park wrote:
> The advertisement of the persistent grants feature (writing
> 'feature-persistent' to xenbus) should mean not the decision for using
> the feature but only the availability of the feature. However, commit
> aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent
> grants") made a field of blkback, which was a place for saving only the
> negotiation result, to be used for yet another purpose: caching of the
> 'feature_persistent' parameter value. As a result, the advertisement,
> which should follow only the parameter value, becomes inconsistent.
>
> This commit fixes the misuse of the semantic by making blkback saves the
> parameter value in a separate place and advertises the support based on
> only the saved value.
>
> Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants")
> Cc: <stable@xxxxxxxxxxxxxxx> # 5.10.x
> Suggested-by: Juergen Gross <jgross@xxxxxxxx>
> Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
> ---
> drivers/block/xen-blkback/common.h | 3 +++
> drivers/block/xen-blkback/xenbus.c | 6 ++++--
> 2 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
> index bda5c815e441..a28473470e66 100644
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -226,6 +226,9 @@ struct xen_vbd {
> sector_t size;
> unsigned int flush_support:1;
> unsigned int discard_secure:1;
> + /* Connect-time cached feature_persistent parameter value */
> + unsigned int feature_gnt_persistent_parm:1;
Continuing over from the previous version:
> > If feature_gnt_persistent_parm is always going to be equal to
> > feature_persistent, then why introduce it at all? Why not just use
> > feature_persistent directly? This way you avoid adding an extra flag
> > whose purpose is not immediately clear, and you also avoid all the
> > mess with setting this flag at the right time.
>
> Mainly because the parameter should read twice (once for
> advertisement, and once later just before the negotitation, for
> checking if we advertised or not), and the user might change the
> parameter value between the two reads.
>
> For the detailed available sequence of the race, you could refer to the
> prior conversation[1].
>
> [1] https://lore.kernel.org/linux-block/20200922111259.GJ19254@Air-de-Roger/
Okay, I see. Thanks for the pointer. But still, I think it would be
better to not maintain two copies of the value. How about doing:
blkif->vbd.feature_gnt_persistent =
xenbus_read_unsigned(dev->nodename, "feature-persistent", 0) &&
xenbus_read_unsigned(dev->otherend, "feature-persistent", 0);
This makes it quite clear that we only enable persistent grants if
_both_ ends support it. We can do the same for blkfront.
> + /* Persistent grants feature negotiation result */
> unsigned int feature_gnt_persistent:1;
> unsigned int overflow_max_grants:1;
> };
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index ee7ad2fb432d..c0227dfa4688 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -907,7 +907,7 @@ static void connect(struct backend_info *be)
> xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
>
> err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
> - be->blkif->vbd.feature_gnt_persistent);
> + be->blkif->vbd.feature_gnt_persistent_parm);
> if (err) {
> xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
> dev->nodename);
> @@ -1085,7 +1085,9 @@ static int connect_ring(struct backend_info *be)
> return -ENOSYS;
> }
>
> - blkif->vbd.feature_gnt_persistent = feature_persistent &&
> + blkif->vbd.feature_gnt_persistent_parm = feature_persistent;
> + blkif->vbd.feature_gnt_persistent =
> + blkif->vbd.feature_gnt_persistent_parm &&
> xenbus_read_unsigned(dev->otherend, "feature-persistent", 0);
>
> blkif->vbd.overflow_max_grants = 0;
> --
> 2.25.1
>
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