linux-kernel - MTD: mtd_blkdevs regression

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Message-ID: <20100910110446.GA28045@angel.research.nokia.com>
Date:	Fri, 10 Sep 2010 14:04:46 +0300
From:	Jarkko Lavinen <jarkko.lavinen@...ia.com>
To:	linux-kernel@...r.kernel.org
Cc:	David Woodhouse <david.woodhouse@...el.com>,
	Maxim Levitsky <maximlevitsky@...il.com>
Subject: MTD: mtd_blkdevs regression

In 2.6.34 modules using add_mtd_blktrans_dev() have reference
count 0 after loading and I can remove the module if it is not
being used. 

In 2.6.34, using mtdblock as an example:

    # modprobe nandsim
    # modprobe mtdblock
    # lsmod 
    Module                  Size  Used by
    mtdblock                2829  0 
    mtd_blkdevs             4217  1 mtdblock
    nandsim                25811  0 
    nand                   27184  1 nandsim
    nand_ids                2666  2 nandsim,nand
    nand_ecc                3392  1 nand
    mtd                    14794  5 mtd_blkdevs,nandsim,nand
    # rmmod mtdblock
    # 

Since commit 048d871 (mtd: blktrans: Hotplug fixes) in 2.6.35-rc1 
add_mtd_blktrans_dev() uses __get_module(tr->owner) to add reference 
count to calling module.

Because of this the module cannot be removed and since module
exit cannot be called, del_mtd_blktrans_dev() will not be called,
which would do the module_put().

In 2.6.35:

    # modprobe nandsim
    # modprobe mtdblock
    # lsmod
    Module                  Size  Used by
    mtdblock                2721  1 
    mtd_blkdevs             4895  1 mtdblock
    nandsim                25819  1 
    nand                   28326  1 nandsim
    nand_ids                2698  2 nandsim,nand
    nand_ecc                3400  1 nand
    mtd                    15352  5 mtd_blkdevs,nandsim,nand
    # rmmod mtdblock
    ERROR: Module mtdblock is in use
    #

Jarkko Lavinen
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