Re: [PATCH] scsi: core: Cap shost cmd_per_lun at can_queue

[John Garry]

On 20/05/2021 16:57, Bart Van Assche wrote:
> On 5/19/21 7:31 AM, John Garry wrote:
> > Function sdev_store_queue_depth() enforces that the sdev queue depth cannot
> > exceed Shost.can_queue.
> >
> > The sdev initial value comes from shost cmd_per_lun.
> >
> > However, the LLDD may still set cmd_per_lun > can_queue, which leads to an
> > initial sdev queue depth greater than can_queue.
> >
> > Such an issue was reported in [0], which caused a hang. That has since
> > been fixed in commit fc09acb7de31 ("scsi: scsi_debug: Fix cmd_per_lun,
> > set to max_queue").
> >
> > Stop this possibly happening for other drivers by capping
> > shost.cmd_per_lun at shost.can_queue.
> >
> > [0] https://lore.kernel.org/linux-scsi/YHaez6iN2HHYxYOh@T590/
> >
> > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> > ---
> > Earlier patch was in https://lore.kernel.org/linux-scsi/1618848384-204144-1-git-send-email-john.garry@xxxxxxxxxx/
> >
> > diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> > index ba72bd4202a2..624e2582c3df 100644
> > --- a/drivers/scsi/hosts.c
> > +++ b/drivers/scsi/hosts.c
> > @@ -220,6 +220,9 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
> >   		goto fail;
> >   	}
> >   
> > +	shost->cmd_per_lun = min_t(short, shost->cmd_per_lun,
> > +				   shost->can_queue);
> > +
> >   	error = scsi_init_sense_cache(shost);
> >   	if (error)
> >   		goto fail;

Hi Bart,

> In SCSI header files a mix of int, short and unsigned int is used for
> cmd_per_lun and can_queue. How about changing the types of these two
> member variables in include/scsi/*h into u16?
I don't mind doing that, but is there any requirement for can_queue to 
not be limited to 16b?

It seems intentional that can_queue is int and cmd_per_lun is short.

As an aside, if short is 16b, it does not even seem to have efficient 
packing on Scsi_host today (although we can move things around):

int can_queue;
short cmd_per_lun;
short unsigned int sg_tablesize;
short unsigned int sg_prot_tablesize;
/* 16b hole */
unsigned int max_sectors;

> Anyway:
>
> Reviewed-by: Bart Van Assche <bvanassche@xxxxxxx>

thanks!

> .