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

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Date:	Sun, 19 Dec 2010 16:38:36 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
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>,
	Mike Anderson <andmike@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, Vasu Dev <vasu.dev@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Ravi Anand <ravi.anand@...gic.com>,
	Andrew Vasquez <andrew.vasquez@...gic.com>,
	Joe Eykholt <jeykholt@...co.com>,
	James Smart <james.smart@...lex.com>,
	Douglas Gilbert <dgilbert@...erlog.com>,
	adam radford <aradford@...il.com>,
	Kashyap Desai <Kashyap.Desai@....com>,
	MPTFusionLinux <DL-MPTFusionLinux@....com>
Subject: Re: [PATCH 01/12] libiscsi: Convert to host_lock less w/
	interrupts disabled internally

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.

> Signed-off-by: Nicholas A. Bellinger <nab@...ux-iscsi.org>
> ---
>  drivers/scsi/libiscsi.c |   10 ++--------
>  1 files changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/scsi/libiscsi.c b/drivers/scsi/libiscsi.c
> index c15fde8..5535782 100644
> --- a/drivers/scsi/libiscsi.c
> +++ b/drivers/scsi/libiscsi.c
> @@ -1599,7 +1599,7 @@ enum {
>  	FAILURE_SESSION_NOT_READY,
>  };
>  
> -static int iscsi_queuecommand_lck(struct scsi_cmnd *sc, void (*done)(struct scsi_cmnd *))
> +int iscsi_queuecommand(struct Scsi_Host *sh, struct scsi_cmnd *sc)
>  {
>  	struct iscsi_cls_session *cls_session;
>  	struct Scsi_Host *host;
> @@ -1609,13 +1609,11 @@ static int iscsi_queuecommand_lck(struct scsi_cmnd *sc, void (*done)(struct scsi
>  	struct iscsi_conn *conn;
>  	struct iscsi_task *task = NULL;
>  
> -	sc->scsi_done = done;
>  	sc->result = 0;
>  	sc->SCp.ptr = NULL;
>  
>  	host = sc->device->host;
>  	ihost = shost_priv(host);
> -	spin_unlock(host->host_lock);
>  
>  	cls_session = starget_to_session(scsi_target(sc->device));
>  	session = cls_session->dd_data;
> @@ -1706,7 +1704,6 @@ static int iscsi_queuecommand_lck(struct scsi_cmnd *sc, void (*done)(struct scsi
>  
>  	session->queued_cmdsn++;
>  	spin_unlock(&session->lock);
> -	spin_lock(host->host_lock);
>  	return 0;
>  
>  prepd_reject:
> @@ -1716,7 +1713,6 @@ reject:
>  	spin_unlock(&session->lock);
>  	ISCSI_DBG_SESSION(session, "cmd 0x%x rejected (%d)\n",
>  			  sc->cmnd[0], reason);
> -	spin_lock(host->host_lock);
>  	return SCSI_MLQUEUE_TARGET_BUSY;
>  
>  prepd_fault:
> @@ -1732,12 +1728,10 @@ fault:
>  		scsi_out(sc)->resid = scsi_out(sc)->length;
>  		scsi_in(sc)->resid = scsi_in(sc)->length;
>  	}
> -	done(sc);
> -	spin_lock(host->host_lock);
> +	sc->scsi_done(sc);
>  	return 0;
>  }
>  
> -DEF_SCSI_QCMD(iscsi_queuecommand)
>  EXPORT_SYMBOL_GPL(iscsi_queuecommand);
>  
>  int iscsi_change_queue_depth(struct scsi_device *sdev, int depth, int reason)
> -- 
> 1.7.3.4
> 
> --
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-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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