Re: [PATCH v2 01/11] scsi: Convert struct Scsi_Host->cmd_serial_number to atomic_t

["Nicholas A. Bellinger" <nab@linux-iscsi.org>]

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@linux-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@linux-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