linux-kernel - Re: [PATCH 1/4] scsi: core: constify pointer to scsi_host

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Message-ID: <4510c5dc-3d7d-fc5f-cb80-34da7dbaaa8e@huawei.com>
Date:   Mon, 9 May 2022 12:28:57 +0100
From:   John Garry <john.garry@...wei.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Christoph Hellwig <hch@...radead.org>
CC:     "Ewan D. Milne" <emilne@...hat.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "Alim Akhtar" <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>,
        "Doug Gilbert" <dgilbert@...erlog.com>,
        <linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <james.smart@...adcom.com>
Subject: Re: [PATCH 1/4] scsi: core: constify pointer to scsi_host_template

On 06/05/2022 17:42, Krzysztof Kozlowski wrote:
>> I don't think scsi_Host is appropriate as this is per-scsi host
>> template, unless you see a way to do it that way. Alternatively we could
>> keep a separate list of registered sht, like this:
>>
>> struct sht_proc_dir {
>> 	int cnt;
>> 	struct list_head list;
>> 	struct proc_dir_entry *proc_dir;
>> 	struct scsi_host_template *sht;
>> };
>> static LIST_HEAD(sht_proc_dir_list);
> Hi everyone,
> 

Hi Krzysztof,

> It took me some time to get back to this topic. I moved the proc_dir out
> of SHT, how John proposed. Patches do not look that bad:
> The commit:
> https://github.com/krzk/linux/commit/157eb2ee8867afbae9dac3836e4c0bedb542e5c1
> 

For some reason I cannot fetch your git due to "error: RPC failed ..." 
which I think is a timeout. I seem to have this problem recently 
whenever a linux.git clone has branches based on linux-next.git . Maybe 
a git config issue for me...

> Branch:
> https://github.com/krzk/linux/commits/n/qcom-ufs-opp-cleanups-v2
> 
> However this does not solve the problem. The SHT has "module" which gets
> incremented/decremented. Exactly like in case of other drivers
> (driver->owner).

Ah, I missed that this could be a problem. So we have this member to 
stop the SCSI host driver being removed when we have disks mounted, etc.

But isn't scsi_host_template.module just a pointer to the local driver 
module data (and that data gets incremented/decremented)? I am looking 
at the THIS_MODULE definition in export.h:

extern stuct module __this_module;
#define THIS_MODULE(&__this_module)

However I do see scsi_device_dev_release(), which does:

sdp->host->hostt->module = NULL

I am not sure how necessary that really is. I would need to check further.

Did you see if there other places which set hostt->module dynamically?

> 
> I started moving the SHT->module to a new field scsi_host->owner and
> trying to use the parent's driver (so PCI, USB) owner.
> I am not sure if it is correct approach, so before implementing such big
> change affecting multiple subsystems (USB, ATA, SCSI) - can you share
> ideas/opinion?
> 
> The Work-in-Progress looks like this (last commit):
> https://github.com/krzk/linux/commit/17609caecd53df20f631703ea084a70e7735b5d7

Thanks,
John