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Message-ID: <33ee1c3a-d37a-b81f-68be-d1901c7dd6e8@acm.org>
Date:   Wed, 28 Sep 2022 11:17:02 -0700
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Subject: Re: [PATCH v5 6/7] module: Improve support for asynchronous module
 exit code
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To:     Luis Chamberlain <mcgrof@kernel.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>
Cc:     "Martin K . Petersen" <martin.petersen@oracle.com>,
        linux-scsi@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        Ming Lei <ming.lei@redhat.com>, Hannes Reinecke <hare@suse.de>,
        John Garry <john.garry@huawei.com>,
        Mike Christie <michael.christie@oracle.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-modules@vger.kernel.org, linux-kernel@vger.kernel.org,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        Tejun Heo <tj@kernel.org>
References: <20220914225621.415631-1-bvanassche@acm.org>
 <20220914225621.415631-7-bvanassche@acm.org>
 <a4084b27-dd2d-10df-493c-35998eed664c@acm.org>
 <YzOPJHSQsPtc5o0Y@bombadil.infradead.org>
From:   Bart Van Assche <bvanassche@acm.org>
In-Reply-To: <YzOPJHSQsPtc5o0Y@bombadil.infradead.org>
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On 9/27/22 17:02, Luis Chamberlain wrote:
> On Tue, Sep 20, 2022 at 10:13:40AM -0700, Bart Van Assche wrote:
>> On 9/14/22 15:56, Bart Van Assche wrote:
>>> Some kernel modules call device_del() from their module exit code and
>>> schedule asynchronous work from inside the .release callback without waiting
>>> until that callback has finished. As an example, many SCSI LLD drivers call
>>> scsi_remove_host() from their module exit code. scsi_remove_host() may
>>> invoke scsi_device_dev_release_usercontext() asynchronously.
>>> scsi_device_dev_release_usercontext() uses the host template pointer and
>>> that pointer usually exists in static storage in the SCSI LLD. Support
>>> using the module reference count to keep the module around until
>>> asynchronous module exiting has completed by waiting in the delete_module()
>>> system call until the module reference count drops to zero.
>>
>> Hi Luis,
>>
>> I'd like to know your opinion about this patch since you are the maintainer
>> of the kernel module system.
> 
> See this patch which extends the documentation of try_module_get():
> 
> https://lkml.kernel.org/r/20211029184500.2821444-7-mcgrof@kernel.org
> 
> You can ignore discussion around the thread as sadly it is just
> irrelevant stuff not about that patch. But the logic it spells out
> is still true.
> 
> So, in short, using try_module_get() on exit is actually the wrong
> thing to do and it is no surprise it would fail. I haven't gotten
> yet around to reviewing Mauro's driver API which let's you unbind
> drivers, but it sounds related so I CC'd you on that.
> 
> So I'd like to ask instead if an alternative to using try_module_get()
> on exit would be better here and for the future.

Hi Luis,

The extended documentation of try_module_get() is very helpful. But 
please note that this patch is not related to try_module_get() at all. 
See also patch 7/7 in this series 
(https://lore.kernel.org/linux-scsi/20220914225621.415631-8-bvanassche@acm.org/).

Thanks,

Bart.