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

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Message-ID: <d01c29c1-bb5a-281d-ef71-9c7b39e28d23@linaro.org>
Date:   Mon, 9 May 2022 15:20:48 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     John Garry <john.garry@...wei.com>,
        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 09/05/2022 13:28, John Garry wrote:
> 
> 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...

Just to be sure - the link was not a git remote, but direct link for a
web browser. The repo is:
https://github.com/krzk/linux.git
branch: 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:

Yes, it is just a pointer, just like 'struct device_driver.owner' is.

> 
> 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 think all SHT set it statically. Then it is being dynamically
incremented when needed and in scsi_device_dev_release() is being
nullified (as you mentioned above).

I guess this SHT->module is actually duplicating what most of drivers
(e.g. PCI, platform, USB) are doing. If I understand correctly, the
Scsi_Host is instantiated by the probe of a driver (pci_driver,
virtio_driver etc), therefore the SHT->module could be simply stored in
Scsi_Host.


>>
>> 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



Best regards,
Krzysztof