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

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Message-Id: <1292808126.20840.33.camel@haakon2.linux-iscsi.org>
Date:	Sun, 19 Dec 2010 17:22:06 -0800
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	James Bottomley <James.Bottomley@...e.de>,
	Jeff Garzik <jeff@...zik.org>, Christoph Hellwig <hch@....de>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Hannes Reinecke <hare@...e.de>,
	Mike Christie <michaelc@...wisc.edu>
Subject: Re: [PATCH 01/12] libiscsi: Convert to host_lock less w/
	interrupts disabled internally

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


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