Re: [PATCH v2] scsi: core: Drop Scsi_Host.default_lock

On 8/6/26 4:39 AM, John Garry wrote:
   > Getting the address of embedded host_lock structure just requires adding
   > a fixed offset value to the shost pointer. However, getting the value of
   > the host_lock pointer requires loading from a fixed offset to the shost
   > pointer. The latter should be very slightly slower, which is relevant as
   > this lock is used a lot throughout the core code and drivers.

Hmm ... SCSI drivers for which performance matters shouldn't use the
SCSI host lock in their hot path.

Any driver which uses DEF_SCSI_QCMD uses this lock in the fastpath. 
However I don't think that many are high performance HBAs. Maybe 
usb/storage/ or pmcraid could be consider higher performing.

Then there is stuff like xen-scsifront.c which uses this lock in the cmd 
done handler. It's used a lot in lpfc and also in code which looks to be 
fastpath...but that drivers is very complex.

   > @Bart, Can you kindly build test this? s390 and sh were causing some
build
   > issues for me. Thanks

This patch passes my build tests, but an update for the references to
default_lock in the Documentation/ directory is missing.

I'll fix it

Otherwise this patch looks good to me. So with or without the
Documentation/ update, feel free to add:

Reviewed-by: Bart Van Assche <bvanassche@acm.org>

thanks