Re: [PATCH] vhost-scsi: Take configfs group dependency during VHOST_SCSI_SET_ENDPOINT

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Hey Paolo,

On Thu, 2014-10-09 at 12:49 +0200, Paolo Bonzini wrote:
> Il 09/10/2014 10:49, Paolo Bonzini ha scritto:
> > 
> > It does not happen if you close QEMU with SIGTERM, ctrl-c, or with the
> > "quit" command, because no attempt is done to bring down the VM data
> > structures (or free memory, or close file descriptors) in case of a
> > fatal exit.  The kernel should do that for us.
> 
> ... and in the case of vhost-scsi, doesn't it do that when
> vhost_scsi_release calls vhost_scsi_clear_endpoint?
> 

Thanks for the extra clarifications here.

The SIGTERM, ctrl-c and "quit" command cases happen as you describe, and
invoke vhost_scsi_release() -> vhost_scsi_clear_endpoint() to drop the
endpoint reference.

AFAICT, it's the SIGKILL case that is problematic both with and without
this patch.  With the patch, the configfs dependency on the vhost-scsi
endpoint group is left in place, thus preventing the group (and
underlying target_core_mod) from being removed until a
VHOST_SCSI_CLEAR_ENDPOINT with the same wwpn is called to drop the
original reference.

Without the patch, the group can still be removed at any time, but any
subsequent VHOST_SCSI_SET_ENDPOINT attempts with the original wwpn will
fail after SIGKILL, because the original reference is still in place.

So I held off on pushing this patch to -rc1 for the moment, but even
with the above limitation preventing group shutdown after SIGKILL, I
think it's still better to obtain the configfs dependency to prevent the
removal of endpoints while there are active references.

That said, I'm still unsure how to address the SIGKILL case, and what's
the most sane way to drop dead references after it happens, and how
vhost-scsi should be differentiating between dead and active references.

Any ideas..?

--nab

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