Re: [PATCH v2] block: End quiescent sections when a BDS is deleted

[Kevin Wolf <kwolf@redhat.com>]

Am 23.10.2020 um 17:01 hat Greg Kurz geschrieben:
> If a BDS gets deleted during blk_drain_all(), it might miss a
> call to bdrv_do_drained_end(). This means missing a call to
> aio_enable_external() and the AIO context remains disabled for
> ever. This can cause a device to become irresponsive and to
> disrupt the guest execution, ie. hang, loop forever or worse.
> 
> This scenario is quite easy to encounter with virtio-scsi
> on POWER when punching multiple blockdev-create QMP commands
> while the guest is booting and it is still running the SLOF
> firmware. This happens because SLOF disables/re-enables PCI
> devices multiple times via IO/MEM/MASTER bits of PCI_COMMAND
> register after the initial probe/feature negotiation, as it
> tends to work with a single device at a time at various stages
> like probing and running block/network bootloaders without
> doing a full reset in-between. This naturally generates many
> dataplane stops and starts, and thus many drain sections that
> can race with blockdev_create_run(). In the end, SLOF bails
> out.
> 
> It is somehow reproducible on x86 but it requires to generate
> articial dataplane start/stop activity with stop/cont QMP
> commands. In this case, seabios ends up looping for ever,
> waiting for the virtio-scsi device to send a response to
> a command it never received.
> 
> Add a helper that pairs all previously called bdrv_do_drained_begin()
> with a bdrv_do_drained_end() and call it from bdrv_close().
> While at it, update the "/bdrv-drain/graph-change/drain_all"
> test in test-bdrv-drain so that it can catch the issue.
> 
> BugId: https://bugzilla.redhat.com/show_bug.cgi?id=1874441
> Signed-off-by: Greg Kurz <groug@kaod.org>

Thanks, applied to the block branch.

Kevin