linux-kernel - Re: [PATCH v2] mmc: inline the first mmc_scan() on mmc_start

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Date:   Fri, 31 Mar 2023 14:43:10 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Dennis Zhou <dennis@...nel.org>
Cc:     linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mmc: inline the first mmc_scan() on mmc_start_host()

On Thu, 30 Mar 2023 at 01:48, Dennis Zhou <dennis@...nel.org> wrote:
>
> When using dm-verity with a data partition on an emmc device, dm-verity
> races with the discovery of attached emmc devices. This is because mmc's
> probing code sets up the host data structure then a work item is
> scheduled to do discovery afterwards. To prevent this race on init,
> let's inline the first call to detection, __mm_scan(), and let
> subsequent detect calls be handled via the workqueue.

In principle, I don't mind the changes in $subject patch, as long as
it doesn't hurt the overall initialization/boot time. Especially, we
may have more than one mmc-slot being used, so this needs to be well
tested.

Although, more importantly, I fail to understand how this is going to
solve the race condition. Any I/O request to an eMMC or SD requires
the mmc block device driver to be up and running too, which is getting
probed from a separate module/driver that's not part of mmc_rescan().

Kind regards
Uffe

>
> Signed-off-by: Dennis Zhou <dennis@...nel.org>
> ---
> Sigh.. fix missing static declaration.
>
>  drivers/mmc/core/core.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> index 368f10405e13..fda7ee57dee3 100644
> --- a/drivers/mmc/core/core.c
> +++ b/drivers/mmc/core/core.c
> @@ -2185,10 +2185,8 @@ int mmc_card_alternative_gpt_sector(struct mmc_card *card, sector_t *gpt_sector)
>  }
>  EXPORT_SYMBOL(mmc_card_alternative_gpt_sector);
>
> -void mmc_rescan(struct work_struct *work)
> +static void __mmc_rescan(struct mmc_host *host)
>  {
> -       struct mmc_host *host =
> -               container_of(work, struct mmc_host, detect.work);
>         int i;
>
>         if (host->rescan_disable)
> @@ -2249,6 +2247,14 @@ void mmc_rescan(struct work_struct *work)
>                 mmc_schedule_delayed_work(&host->detect, HZ);
>  }
>
> +void mmc_rescan(struct work_struct *work)
> +{
> +       struct mmc_host *host =
> +               container_of(work, struct mmc_host, detect.work);
> +
> +       __mmc_rescan(host);
> +}
> +
>  void mmc_start_host(struct mmc_host *host)
>  {
>         host->f_init = max(min(freqs[0], host->f_max), host->f_min);
> @@ -2261,7 +2267,8 @@ void mmc_start_host(struct mmc_host *host)
>         }
>
>         mmc_gpiod_request_cd_irq(host);
> -       _mmc_detect_change(host, 0, false);
> +       host->detect_change = 1;
> +       __mmc_rescan(host);
>  }
>
>  void __mmc_stop_host(struct mmc_host *host)
> --
> 2.40.0
>