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

From: Ming Lei <ming.lei@redhat.com>
To: Bart Van Assche <bvanassche@acm.org>
Cc: "Martin K . Petersen" <martin.petersen@oracle.com>,
	linux-scsi@vger.kernel.org, Luis Chamberlain <mcgrof@kernel.org>,
	Christoph Hellwig <hch@lst.de>, 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>,
	ming.lei@redhat.com
Subject: Re: [PATCH v5 6/7] module: Improve support for asynchronous module exit code
Date: Wed, 28 Sep 2022 09:09:50 +0800	[thread overview]
Message-ID: <YzOe3pYmn5qO9lFb@T590> (raw)
In-Reply-To: <20220914225621.415631-7-bvanassche@acm.org>

On Wed, Sep 14, 2022 at 03:56:20PM -0700, 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

It isn't only related with device, any kobject has such issue, or any
reference counter usage has similar potential risk, see previous discussion:

https://lore.kernel.org/lkml/YsZm7lSXYAHT14ui@T590/

IMO, it is one fundamental problem wrt. module vs. reference counting or
kobject uses at least, since the callback depends on module code
segment.

> 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.

The issue can't be addressed by the normal mod->refcnt, since user need
to unload module when the device isn't used.

thanks,
Ming