Re: [PATCH 01/12] libiscsi: Convert to host_lock less w/ interrupts disabled internally

Nicholas A. Bellinger
2010-12-20T01:28:06Z

On Sun, 2010-12-19 at 17:15 -0800, Nicholas A. Bellinger wrote:
> On Sun, 2010-12-19 at 16:38 -0700, Matthew Wilcox wrote:
> > On Sun, Dec 19, 2010 at 01:21:56PM -0800, Nicholas A. Bellinger wrote:
> > > This patch changes iscsi_queuecommand_lck() to a host_lock less
> > > iscsi_queuecommand() that will internally disable interrupts using
> > > session->lock and drop the now legacy host_lock unlock.
> >
> > I think this patch is buggy. Before, iscsi_queuecommand_lck is called with
> > interrupts disabled. Now, it's called with interrupts disabled. Elsewhere,
> > session->lock is acquired with bh's disabled:
> >
> > void iscsi_put_task(struct iscsi_task *task)
> > {
> > struct iscsi_session *session = task->conn->session;
> >
> > spin_lock_bh(&session->lock);
> >
> > So I think you need to convert the
> > spin_lock(&session->lock);
> > in iscsi_queuecommand_lck to at least a spin_lock_bh, and possibly
> > a spin_lock_irq -- it's not clear to me whether it needs to exclude
> > against interrupt context, or only BH context. In some places, it's
> > taken with spin_lock_irqsave().
> >
> > I'd play it safe and use _irq, but someone more confident with this code
> > might choose to only use _bh.
> >
>
> Hmmm, indeed.. I must have dropped this recently as the last patch for
> libiscsi posted below still has been converted to spin_lock_irq() for
> session->lock here:
>
> http://marc.info/?l=linux-scsi&m=128952081412461&w=2
>
> Converting back to spin_lock_irq() usage for the moment.. Thanks alot
> of noticing this..
>
> MikeC and Hannes, do you think this is safe to use spin_lock_bh() as
> well..?
>

Actually sorry, Mike Christie did already make a clarification on this
subject here:

http://marc.info/?l=linux-scsi&m=129010439421506&w=2

I had originally thought the same that session->lock should be using
some flavour of spin_lock_irq*() as well, but apparently this is not the
case for libiscsi.

Best Regards,

--nab