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Date:   Fri, 31 Mar 2023 11:23:12 -0700
From:   Dennis Zhou <dennis@kernel.org>
To:     Ulf Hansson <ulf.hansson@linaro.org>
Cc:     linux-mmc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mmc: inline the first mmc_scan() on mmc_start_host()
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References: <20230329202148.71107-1-dennis@kernel.org>
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Hi Ulf,

On Fri, Mar 31, 2023 at 02:43:10PM +0200, Ulf Hansson wrote:
> On Thu, 30 Mar 2023 at 01:48, Dennis Zhou <dennis@kernel.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.
> 

I unfortunately don't have a device with multiple mmcs available. Is
this something you could help me with?

> 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().

I believe the call chain is something like this:

__mmc_rescan()
    mmc_rescan_try_freq()
        mmc_attach_mmc()
            mmc_add_card()
                device_add()
                    bus_probe_device()
                        mmc_blk_probe()

The initial calling of this is the host probe. So effectively if there
is a card attached, we're inlining the device_add() call for the card
attached rather than waiting for the workqueue item to kick off.

dm is a part of late_initcall() while mmc is a module_init(), when built
in becoming a device_initcall(). So this solves a race via the initcall
chain. In the current state, device_initcall() finishes and we move onto
the late_initcall() phase. But now, dm is racing with the workqueue to
init the attached emmc device.

Thanks,
Dennis

> 
> Kind regards
> Uffe
> 
> >
> > Signed-off-by: Dennis Zhou <dennis@kernel.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
> >