linux-kernel - Re: [PATCH 17/32] scsi/ibmvstgt: use system

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Date:	Mon, 24 Jan 2011 17:09:18 +0100
From:	Bart Van Assche <bvanassche@....org>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	"James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
	linux-scsi@...r.kernel.org, Brian King <brking@...ux.vnet.ibm.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>
Subject: Re: [PATCH 17/32] scsi/ibmvstgt: use system_wq instead of vtgtd workqueue

On Mon, Jan 3, 2011 at 2:49 PM, Tejun Heo <tj@...nel.org> wrote:
>
> The target driver is not in the memory reclaim path and doesn't need a
> dedicated workqueue.  Drop vtgtd and use system_wq instead.  The used
> work item is sync flushed on removal.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
> Cc: "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>
> Cc: linux-scsi@...r.kernel.org
> ---
> Only compile tested.  Please feel free to take it into the subsystem
> tree or simply ack - I'll route it through the wq tree.
>
> Thanks.
>
>  drivers/scsi/ibmvscsi/ibmvstgt.c |   15 ++++-----------
>  1 files changed, 4 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/scsi/ibmvscsi/ibmvstgt.c b/drivers/scsi/ibmvscsi/ibmvstgt.c
> index 2256bab..47fc632 100644
> --- a/drivers/scsi/ibmvscsi/ibmvstgt.c
> +++ b/drivers/scsi/ibmvscsi/ibmvstgt.c
> @@ -74,7 +74,6 @@ struct vio_port {
>        struct srp_rport *rport;
>  };
>
> -static struct workqueue_struct *vtgtd;
>  static struct scsi_transport_template *ibmvstgt_transport_template;
>
>  /*
> @@ -546,7 +545,7 @@ static irqreturn_t ibmvstgt_interrupt(int dummy, void *data)
>        struct vio_port *vport = target_to_port(target);
>
>        vio_disable_interrupts(vport->dma_dev);
> -       queue_work(vtgtd, &vport->crq_work);
> +       schedule_work(&vport->crq_work);
>
>        return IRQ_HANDLED;
>  }
> @@ -900,6 +899,7 @@ static int ibmvstgt_remove(struct vio_dev *dev)
>        crq_queue_destroy(target);
>        srp_remove_host(shost);
>        scsi_remove_host(shost);
> +       flush_work_sync(&vport->crq_work);
>        scsi_tgt_free_queue(shost);
>        srp_target_free(target);
>        kfree(vport);
> @@ -967,21 +967,15 @@ static int __init ibmvstgt_init(void)
>        if (!ibmvstgt_transport_template)
>                return err;
>
> -       vtgtd = create_workqueue("ibmvtgtd");
> -       if (!vtgtd)
> -               goto release_transport;
> -
>        err = get_system_info();
>        if (err)
> -               goto destroy_wq;
> +               goto release_transport;
>
>        err = vio_register_driver(&ibmvstgt_driver);
>        if (err)
> -               goto destroy_wq;
> +               goto release_transport;
>
>        return 0;
> -destroy_wq:
> -       destroy_workqueue(vtgtd);
>  release_transport:
>        srp_release_transport(ibmvstgt_transport_template);
>        return err;
> @@ -991,7 +985,6 @@ static void __exit ibmvstgt_exit(void)
>  {
>        printk("Unregister IBM virtual SCSI driver\n");
>
> -       destroy_workqueue(vtgtd);
>        vio_unregister_driver(&ibmvstgt_driver);
>        srp_release_transport(ibmvstgt_transport_template);
>  }

(added Brian King and Robert Jennings in CC)

Hello Tejun,

Insertion of  flush_work_sync() fixes a race - that's a good catch.
flush_work_sync() should be invoked a little earlier though because
the scheduled work may access the queue destroyed by the
crq_queue_destroy(target) call. And the CRQ interrupt should be
disabled from before flush_work_sync() is invoked until after the CRQ
has been destroyed.

Regarding the queue removal: I might have missed something, but why
would you like to remove the vtgtd work queue ? Since the ibmvstgt
driver is a storage target driver, processing latency matters. I'm
afraid that switching from a dedicated queue to the global work queue
will increase processing latency.

Bart.
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