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Date: Fri, 17 Sep 2010 12:33:43 -0700
From: "Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To: Joe Eykholt <jeykholt@...co.com>
Cc: linux-scsi <linux-scsi@...r.kernel.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Vasu Dev <vasu.dev@...ux.intel.com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Andi Kleen <ak@...ux.intel.com>,
Matthew Wilcox <willy@...ux.intel.com>,
James Bottomley <James.Bottomley@...e.de>,
Mike Christie <michaelc@...wisc.edu>,
James Smart <james.smart@...lex.com>,
Andrew Vasquez <andrew.vasquez@...gic.com>,
FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
Hannes Reinecke <hare@...e.de>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 01/11] scsi: Convert struct
Scsi_Host->cmd_serial_number to atomic_t
On Fri, 2010-09-17 at 12:03 -0700, Joe Eykholt wrote:
> On 9/17/10 11:21 AM, Nicholas A. Bellinger wrote:
> > From: Nicholas Bellinger <nab@...ux-iscsi.org>
> >
> > This patch converts struct Scsi_Host->cmd_serial_number to an atomic_t so that
> > scsi_cmd_get_serial() can be safely called without struct Scsi_Host->host_lock
> > in scsi_dispatch_cmd(). This patch also adds a struct Scsi_Host->use_serial_number
> > to signal serial_number usage, but this is now disabled by default in
> > scsi_host_alloc().
> >
> > One special item in scsi_cmd_get_serial() recommended by Joe Eykholt is to
> > start struct Scsi_Host->cmd_serial_number at 1, and increment each serial_number
> > by 2 so that the serial is odd, and wraps to 1 instead of 0.
> >
> > Signed-off-by: Nicholas A. Bellinger <nab@...ux-iscsi.org>
> > ---
> > drivers/scsi/hosts.c | 4 ++++
> > drivers/scsi/scsi.c | 13 ++++++++++---
> > include/scsi/scsi_host.h | 8 +++++---
> > 3 files changed, 19 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> > index 8a8f803..aff1c9c 100644
> > --- a/drivers/scsi/hosts.c
> > +++ b/drivers/scsi/hosts.c
> > @@ -380,6 +380,10 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
> > shost->unchecked_isa_dma = sht->unchecked_isa_dma;
> > shost->use_clustering = sht->use_clustering;
> > shost->ordered_tag = sht->ordered_tag;
> > + /*
> > + * Set the default shost->cmd_serial_number to 1.
> > + */
>
> Comment not necessary.
<nod>
>
> > + atomic_set(&shost->cmd_serial_number, 1);
> >
> > if (sht->supported_mode == MODE_UNKNOWN)
> > /* means we didn't set it ... default to INITIATOR */
> > diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
> > index ad0ed21..4724ce7 100644
> > --- a/drivers/scsi/scsi.c
> > +++ b/drivers/scsi/scsi.c
> > @@ -636,9 +636,16 @@ void scsi_log_completion(struct scsi_cmnd *cmd, int disposition)
> > */
> > static inline void scsi_cmd_get_serial(struct Scsi_Host *host, struct scsi_cmnd *cmd)
> > {
> > - cmd->serial_number = host->cmd_serial_number++;
> > - if (cmd->serial_number == 0)
> > - cmd->serial_number = host->cmd_serial_number++;
> > + /*
> > + * The use of per struct scsi_cmnd->serial_number is disabled by default
> > + */
> > + if (!(host->use_serial_number))
> > + return;
> > + /*
> > + * Increment the host->cmd_serial_number by 2 so cmd->serial_number
> > + * is always odd and wraps to 1 instead of 0.
> > + */
> > + cmd->serial_number = atomic_add_return(2, &host->cmd_serial_number);
> > }
> >
> > /**
> > diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> > index b7bdecb..b08d0f2 100644
> > --- a/include/scsi/scsi_host.h
> > +++ b/include/scsi/scsi_host.h
> > @@ -602,10 +602,9 @@ struct Scsi_Host {
> > short unsigned int max_sectors;
> > unsigned long dma_boundary;
> > /*
> > - * Used to assign serial numbers to the cmds.
> > - * Protected by the host lock.
> > + * Used to assign serial numbers to the cmds in scsi_cmd_get_serial()
> > */
> > - unsigned long cmd_serial_number;
> > + atomic_t cmd_serial_number;
> >
> > unsigned active_mode:2;
> > unsigned unchecked_isa_dma:1;
> > @@ -636,6 +635,9 @@ struct Scsi_Host {
> > /* Asynchronous scan in progress */
> > unsigned async_scan:1;
> >
> > + /* Signal usage of per struct scsi_cmnd->serial_number */
> > + unsigned use_serial_number:1;
> > +
> > /*
> > * Optional work queue to be utilized by the transport
> > */
>
> Simple code is so much fun to critique! So here goes:
>
> This patch would break any drivers that care about serial_numbers
> because it never sets use_serial_number in any drivers. So that should
> be done in the same patch so it's bisect-able.
>
<nod>
> How about instead of adding use_serial_number, let's just have the
> drivers that want a serial number call scsi_cmd_get_serial()
> and stop calling it from scsi_dispatch_cmd()?
This makes sense to me...
> AFAICT, it's only
> used in debug messages in some drivers. I didn't find other usages
> but didn't do an exhaustive search.
This is the same conclusion that I reached when I had a first glance,
that ->serial_number is only used for informational purposes in about
1/2 dozen or so mostly old and semi-obsecure LLDs.
James and Co, would this be acceptable to move scsi_cmd_get_serial()
directly into LLD SHT->queuecommand() callers for the old drivers that
still use struct scsi_cmnd->serial_number for whatever purposes..?
> If the drivers do it themselves,
> maybe they're already under a lock when they get the serial number
> and then we wouldn't need the atomic.
>
Yeah, not sure on this part yet..
> Thanks for using my increment by 2 idea.
Thanks Joe!
--nab
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