Race-free NAND device removal

[Boris Brezillon]

On Sun, 3 Jul 2016 15:38:42 +0200
Richard Weinberger <richard at nod.at> wrote:

> Hi!
> 
> While working on nandsim I realized that nand_release() ignores the return
> value from mtd_device_unregister().
> 
> That means NAND devices cannot removed in a race-free manner.
> Consider a NAND driver that registers ->_get_device() and ->_put_device()
> callbacks for refcounting. In its removal function it will return -EBUSY
> whenever the refcount is > 0.
> But when device is claimed while removing it, it can happen that the refcount
> increments after the check.
> MTD can deal with that and mtd_device_unregister() will return EBUSY.
> But nand_release() won't notice and the NAND driver continues with the tear down
> process.

Yes, I already noticed that, and apparently all NAND controller drivers
seem to assume that nand_release() always succeed. It's definitely a
bug, since the MTD device will still be exposed, but the underlying
NAND structure (and the associated data + implementation) will be
gone :-/.

> 
> Would be a change like the following one acceptable or is a NAND driver
> allowed to call mtd_device_unregister() itself?
> AFAICT the additional call to mtd_device_unregister() in nand_release() would
> be an nop then.

This patch looks good, but NAND controller drivers will keep ignoring
the nand_release() return code and release their own private data, so
implementations are still buggy ;).

This whole NAND dev registration/deregistration is unsafe, and I plan
to rework it when moving to a controller <-> chips infrastructure.

Are you fixing a real bug or just a potential one? Cause I'm not sure
doing that is any safer if we don't patch all the NAND controller
drivers...

> 
> diff --git a/drivers/mtd/nand/nand_base.c b/drivers/mtd/nand/nand_base.c
> index 0b0dc29..dc76bc6 100644
> --- a/drivers/mtd/nand/nand_base.c
> +++ b/drivers/mtd/nand/nand_base.c
> @@ -4604,16 +4604,19 @@ EXPORT_SYMBOL(nand_scan);
>   * nand_release - [NAND Interface] Free resources held by the NAND device
>   * @mtd: MTD device structure
>   */
> -void nand_release(struct mtd_info *mtd)
> +int nand_release(struct mtd_info *mtd)
>  {
> +	int ret;
>  	struct nand_chip *chip = mtd_to_nand(mtd);
> 
> +	ret = mtd_device_unregister(mtd);
> +	if (ret)
> +		return ret;
> +
>  	if (chip->ecc.mode == NAND_ECC_SOFT &&
>  	    chip->ecc.algo == NAND_ECC_BCH)
>  		nand_bch_free((struct nand_bch_control *)chip->ecc.priv);
> 
> -	mtd_device_unregister(mtd);
> -
>  	/* Free bad block table memory */
>  	kfree(chip->bbt);
>  	if (!(chip->options & NAND_OWN_BUFFERS))
> @@ -4623,6 +4626,8 @@ void nand_release(struct mtd_info *mtd)
>  	if (chip->badblock_pattern && chip->badblock_pattern->options
>  			& NAND_BBT_DYNAMICSTRUCT)
>  		kfree(chip->badblock_pattern);
> +
> +	return 0;
>  }
>  EXPORT_SYMBOL_GPL(nand_release);
> 
> diff --git a/include/linux/mtd/nand.h b/include/linux/mtd/nand.h
> index fbe8e16..c15b1c4 100644
> --- a/include/linux/mtd/nand.h
> +++ b/include/linux/mtd/nand.h
> @@ -39,7 +39,7 @@ extern int nand_scan_ident(struct mtd_info *mtd, int max_chips,
>  extern int nand_scan_tail(struct mtd_info *mtd);
> 
>  /* Free resources held by the NAND device */
> -extern void nand_release(struct mtd_info *mtd);
> +extern int nand_release(struct mtd_info *mtd);
> 
>  /* Internal helper for board drivers which need to override command function */
>  extern void nand_wait_ready(struct mtd_info *mtd);
> 
> Thanks,
> //richard