Re: [PATCH v2] dm init: ensure block device is ready before creating mapped device

[Sasha Levin]

On Fri, Dec 12, 2025 at 12:09:55AM +0000, jaeyuel.im@xxxxxxx wrote:
> From: "jaeyuel.im" <jaeyuel.im@xxxxxxx>
>
> The current implementation of dm_init_init() uses early_lookup_bdev() to
> wait for the device node to appear. However, early_lookup_bdev() only
> verifies that the device node exists and returns the dev_t. It does not
> guarantee that the underlying block device structure is fully initialized
> and ready for I/O operations or to be opened.
>
> On certain platforms (e.g., embedded systems with specific storage
> drivers), this can lead to a race condition where dm_early_create()
> attempts to open the device immediately after early_lookup_bdev() returns,
> but fails because the device is not yet fully ready. This results in boot
> failures as the mapped device cannot be created.
>
> This patch adds an additional check using blkdev_get_no_open() after
> early_lookup_bdev() returns. This ensures that the struct block_device is
> actually available and the device is ready to be opened, effectively
> preventing the race condition.
>
> Changes in v2:
> - Pass autoload parameter for new blkdev_get_no_open()
>
> Signed-off-by: jaeyuel.im <jaeyuel.im@xxxxxxx>
> ---
> drivers/md/dm-init.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/drivers/md/dm-init.c b/drivers/md/dm-init.c
> index b37bbe762500..b3905e094ffc 100644
> --- a/drivers/md/dm-init.c
> +++ b/drivers/md/dm-init.c
> @@ -296,10 +296,24 @@ static int __init dm_init_init(void)
> 	for (i = 0; i < ARRAY_SIZE(waitfor); i++) {
> 		if (waitfor[i]) {
> 			dev_t dev;
> +			struct block_device *bdev;
>
> 			DMINFO("waiting for device %s ...", waitfor[i]);
> 			while (early_lookup_bdev(waitfor[i], &dev))
> 				fsleep(5000);
> +
> +			/*
> +			 * early_lookup_bdev() only checks if the device node exists and
> +			 * returns the dev_t. It does not guarantee that the underlying
> +			 * block device is fully initialized and ready to be opened. On
> +			 * some platforms, this can lead to a race condition where
> +			 * dm_early_create() fails because the device is not yet ready.
> +			 * Ensure the block device is truly available by attempting to
> +			 * get it.
> +			 */
> +			while (!(bdev = blkdev_get_no_open(dev, false)))

This breaks the build when CONFIG_DM_INIT=y:

  drivers/md/dm-init.c:314:41: error: implicit declaration of function 'blkdev_get_no_open'; did you mean 'blkdev_get_zone_info'? [-Wimplicit-function-declaration]
    314 |                         while (!(bdev = blkdev_get_no_open(dev, false)))
  drivers/md/dm-init.c:316:25: error: implicit declaration of function 'blkdev_put_no_open' [-Wimplicit-function-declaration]
    316 |                         blkdev_put_no_open(bdev);

The problem is that blkdev_get_no_open() and blkdev_put_no_open() are declared
in block/blk.h, which is internal to the block layer and not accessible from
drivers/md/. These functions need to be exported to a public header, or
dm-init.c needs to use a different API.

--
Thanks,
Sasha