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

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Date:	Mon, 20 Dec 2010 01:30:13 -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>,
	Open iSCSI <open-iscsi@...glegroups.com>
Subject: Re: [PATCH 01/12] libiscsi: Convert to host_lock less w/
	interrupts disabled internally

On Sun, 2010-12-19 at 19:07 -0700, Matthew Wilcox wrote:
> On Sun, Dec 19, 2010 at 05:22:06PM -0800, Nicholas A. Bellinger wrote:
> > 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.
> 
> Right, so it seems.  "the session lock is just locked in softirqs/timers"
> means that it does need to be the _bh() version of spin_lock though.
> 
> I'm actually not sure ... is it safe to use the _bh versions in BH
> context?  I think it is because the preempt count is nested, unlike the
> _irq variants of spinlocks.
> 

Hmmm, fair point.  Merging the following incremental patch to convert
session->lock to use spin_lock_bh() in iscsi_queuecommand() into
lock_less-LLDs-for-38-v3:

commit 744f1c119b3fb0c1a6b3c67a7b490e234d1a7e75
Author: Nicholas Bellinger <nab@...ux-iscsi.org>
Date:   Mon Dec 20 09:17:19 2010 +0000

    libiscsi: Convert iscsi_queuecommand to use spin_lock_bh
    
    This patch converts iscsi_queuecommand() code to obtain struct iscsi_session->lock
    using spin_lock_bh() to properly handle bottom-half context operation.
    
    Reported-by: Matthew Wilcox <willy@...ux.intel.com>
    Signed-off-by: Nicholas A. Bellinger <nab@...ux-iscsi.org>

After a quick audit of iscsi_session->lock usage, and I see that
iscsi_complete_pdu(), iscsi_tmf_timedout(), iscsi_eh_cmd_timed_out(),
iscsi_check_transport_timeouts() are using spin_lock(), and
iscsi_session_failure() and iscsi_conn_failure() are using
spin_lock_irqsave().

Mike and Hannes, would you guys mind commenting on this..?  From what I
can determine these should all be converted to use spin_lock_bh(),
yes..?

--nab


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