Re: [PATCH] libfc: fix seconds_since_last_reset miscalculation

[Bart Van Assche]

On 11/08/16 00:45, Johannes Thumshirn wrote:
> Commit 540eb1eef 'scsi: libfc: fix seconds_since_last_reset calculation'
> removed the use of 'struct timespec' from fc_get_host_stats(). This broke the
> output of 'fcoeadm -s' after kernel 4.8-rc1 as lport->boot_time - jiffies
> could become negative as in this example:
>
> $ cat /sys/class/fc_host/host8/statistics/seconds_since_last_reset
> 0x10624dd2f1977b4
>
> Take this into account so
> /sys/class/fc_host/hostX/statistics/seconds_since_last_reset is sane again.
>
> Fixes: 540eb1eef ('scsi: libfc: fix seconds_since_last_reset calculation')
> Signed-off-by: Johannes Thumshirn <jthumshirn@xxxxxxx>
> Tested-by: Holger Schranz <holger@xxxxxxxxxxxxxx>
> ---
>  drivers/scsi/libfc/fc_lport.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/scsi/libfc/fc_lport.c b/drivers/scsi/libfc/fc_lport.c
> index 04ce7cf..475c0a9 100644
> --- a/drivers/scsi/libfc/fc_lport.c
> +++ b/drivers/scsi/libfc/fc_lport.c
> @@ -304,11 +304,15 @@ struct fc_host_statistics *fc_get_host_stats(struct Scsi_Host *shost)
>  	unsigned int cpu;
>  	u64 fcp_in_bytes = 0;
>  	u64 fcp_out_bytes = 0;
> +	unsigned long boot_time = lport->boot_time;
>
>  	fc_stats = &lport->host_stats;
>  	memset(fc_stats, 0, sizeof(struct fc_host_statistics));
>
> -	fc_stats->seconds_since_last_reset = (lport->boot_time - jiffies) / HZ;
> +	if (boot_time > jiffies)
> +		fc_stats->seconds_since_last_reset = (boot_time - jiffies) / HZ;
> +	else
> +		fc_stats->seconds_since_last_reset = (jiffies - boot_time) / HZ;
>
>  	for_each_possible_cpu(cpu) {
>  		struct fc_stats *stats;

Hello Johannes,

I think the above code will miscalculate seconds_since_last_reset if 
'jiffies' wraps around after an lport has been created and before 
seconds_since_last_reset is computed. Shouldn't seconds_since_last_reset 
be computed as follows?

	fc_stats->seconds_since_last_reset = (jiffies - boot_time) / HZ;

Bart.