Re: [PATCH v5 6/7] module: Improve support for asynchronous module exit code

Bart Van Assche
2022-09-28T18:18:49Z

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.