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

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Message-ID: <fd2cdc06-5d88-306a-3ee1-7aef3e3b8921@huawei.com>
Date:   Mon, 9 May 2022 15:50:33 +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 09/05/2022 14:20, Krzysztof Kozlowski wrote:
> 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
> 

OK, I'll double check. Regardless I'll sort my own IT issues :)

>>> 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. 

Incidentally I did notice that the ata ahci driver does not set sht->module.

> 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.

If you check scsi_device_dev_release(), we try to do a 'get' - if it 
fails, then we nullify hostt->module. I think that is important as then 
we call execute_in_process_context(), whose worker does the 'put'. 
However, the 'put' will get upset if the refcnt was 0, which it would be 
if the earlier 'get' fails - hence the nullify is to avoid that 
possibility. So whatever you do needs to handle that. Details are in 
f2b85040

Thanks,
john