Re: [PATCH 0/3] scsi: infinite guest hangs with scsi-disk

[Paolo Bonzini]

On 16/11/20 19:31, Hannes Reinecke wrote:
> Hi all,
>
> one of our customers reported an infinite guest hang following an FC link loss  
> when using scsi-disk.
> Problem is that scsi-disk issues SG_IO command with a timeout of UINT_MAX, 
> which essentially signals
> 'no timeout' to the host kernel. So if the command gets lost eg during an 
> unexpected link loss the
> HBA driver will never attempt to abort or return the command. Hence the guest 
> will hang forever, and
> the only way to resolve things is to reboot the host.
>
> To solve it this patchset adds an 'io_timeout' parameter to scsi-disk and 
> scsi-generic, which allows
> the admin to specify a command timeout for SG_IO request. It is initialized to 
> 30 seconds to avoid the
> infinite hang as mentioned above.
>
> As usual, comments and reviews are welcome.
>
> Hannes Reinecke (3):
>    virtio-scsi: trace events
>    scsi: make io_timeout configurable
>    scsi: add tracing for SG_IO commands
>
>   hw/scsi/scsi-disk.c    |  9 ++++++---
>   hw/scsi/scsi-generic.c | 25 ++++++++++++++++++-------
>   hw/scsi/trace-events   | 13 +++++++++++++
>   hw/scsi/virtio-scsi.c  | 30 +++++++++++++++++++++++++++++-
>   include/hw/scsi/scsi.h |  4 +++-
>   5 files changed, 69 insertions(+), 12 deletions(-)

The UINT_MAX timeout predates me, but I think the idea was to make it 
sort of like NFS's hard option.  Without a timeout you cannot be quite 
sure if/when the command will stay in some buffer of the HBA or the SAN 
or the target, and there could be unintended reordering of writes.

Though I guess at some point you'll anyway restart the VM on another 
host and the same reordering can happen, so I've queued the patch.

Paolo