On 9/17/10 11:21 AM, Nicholas A. Bellinger wrote:
> From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx>
> ---
> 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.
> + 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.
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()? AFAICT, it's only
used in debug messages in some drivers. I didn't find other usages
but didn't do an exhaustive search. 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.
Thanks for using my increment by 2 idea.
Joe
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