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

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Message-ID: <CAPDyKFqc33gUYXpY==jbNrOiba2_xUYLs-bv0RTYYU5d8T0VBA@mail.gmail.com>
Date:   Mon, 3 Apr 2023 11:50:41 +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 Fri, 31 Mar 2023 at 20:23, Dennis Zhou <dennis@...nel.org> wrote:
>
> 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@...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.
> >
>
> I unfortunately don't have a device with multiple mmcs available. Is
> this something you could help me with?

Yes, I can help to test. Allow me a few days to see what I can do.

Note that, just having one eMMC and one SD card should work too. It
doesn't have to be multiple eMMCs.

>
> > 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.

You certainly have a point!

This should work when the mmc blk module is built-in. Even if that
doesn't solve the entire problem, it should be a step in the right
direction.

I will give it some more thinking and run some tests at my side, then
I will get back to you again.

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
> > >