Re: [PATCH 2/4] scsi: fcoe: Use per-CPU API to update per-CPU statistics.

[Davidlohr Bueso <dave@stgolabs.net>]

On Fri, 06 May 2022, Sebastian Andrzej Siewior wrote:

>The per-CPU statistics (struct fc_stats) is updated by getting a stable
>per-CPU pointer via get_cpu() + per_cpu_ptr() and then performing the
>increment. This can be optimized by using this_cpu_*() which will do
>whatever is needed on the architecture to perform the update safe and
>efficient.
>The read out of the individual value (fc_get_host_stats()) should be
>done by using READ_ONCE() instead of a plain-C access. The difference is
>that READ_ONCE() will always perform a single access while the plain-C
>access can be splitt by the compiler into two loads if it appears
>beneficial.
>The usage of u64 has the side-effect that it is also 64bit wide on 32bit
>architectures and the read is always split into two loads. The can lead
>to strange values if the read happens during an update which alters both
>32bit parts of the 64bit value. This can be circumvanted by either using
>a 32bit variables on 32bit architecures or extending the statistics with
>a sequence counter.
>
>Use this_cpu_*() API to update the statistics and READ_ONCE() to read
>it.

LGTM, feel free to add my:

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>