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Date:   Fri, 9 Jun 2023 00:16:19 -0700
From:   Dennis Zhou <dennis@kernel.org>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Dennis Zhou <dennis@kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        linux-mmc@vger.kernel.org, linux-kernel@vger.kernel.org,
        Linus Walleij <linus.walleij@linaro.org>
Subject: Re: [PATCH v2] mmc: inline the first mmc_scan() on mmc_start_host()
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Hi Greg,

On Fri, Jun 09, 2023 at 08:19:51AM +0200, Greg KH wrote:
> On Thu, Jun 08, 2023 at 01:49:00PM -0700, Dennis Zhou wrote:
> > On Fri, May 12, 2023 at 01:42:51PM +0200, Ulf Hansson wrote:
> > > + Linus,
> > > 
> > > Hi Dennis,
> > > 
> > > On Mon, 3 Apr 2023 at 11:50, Ulf Hansson <ulf.hansson@linaro.org> wrote:
> > > >
> > > > On Fri, 31 Mar 2023 at 20:23, Dennis Zhou <dennis@kernel.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@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?
> > > >
> > > > 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@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
> > > > > > >
> > > 
> > > My apologies for the long delay. I finally managed to test this.
> > > 
> > > I decided to pick an old arm32 based platform. An ST-Ericsson HREF,
> > > based upon the ux500 SoC. It's quite good to use for these types of
> > > tests as it has two eMMCs soldered, an embedded SDIO (for WiFi) and an
> > > SD-card slot. So in total there are 4 devices that get probed.
> > > 
> > > The SDIO card isn't detected properly, but always fails in the similar
> > > way (thus I left it out from the below data). I tested both with and
> > > without an SD card inserted during boot, to get some more data to
> > > compare. These are the summary from my tests:
> > > 
> > > v6.4-rc1 without SD card:
> > > ~2.18s - MMC1 (eMMC)
> > > ~3.33s - MMC3 (eMMC)
> > > ~5.91s - kernel boot complete
> > > 
> > > v6.4-rc1 with an SD card:
> > > ~2.18s - MMC1 (eMMC)
> > > ~3.45s - MMC3 (eMMC)
> > > ~3.57s - MMC2 (SD)
> > > ~5.76s - kernel boot complete
> > > 
> > > v6.4-rc1 + patch without SD card:
> > > ~2.24s - MMC1 (eMMC)
> > > ~3.58s - MMC3 (eMMC)
> > > ~5.96s - kernel boot complete
> > > 
> > > v6.4-rc1 + patch with an SD card:
> > > ~2.24s - MMC1 (eMMC)
> > > ~3.73s - MMC2 (SD)
> > > ~3.98s - MMC3 (eMMC)
> > > ~6.73s - kernel boot complete
> > > 
> > > By looking at these results, I was kind of surprised. I was thinking
> > > that the asynchronous probe should address the parallelism problem.
> > > Then I discovered that it in fact, hasn't been enabled for the mmci
> > > driver that is being used for this platform. Huh, I was under the
> > > assumption that it has been enabled for all mmc hosts by now. :-)
> > > 
> > > Okay, so I am going to run another round of tests, with async probe
> > > enabled for the mmci driver too. I will let you know the results as
> > > soon as I can.
> > > 
> > > Kind regards
> > > Uffe
> > 
> > Hi Uffe,
> > 
> > Kindly this has been way too long for review. It's been over 3 months.
> > What's going on here?
> > 
> > I think there's a misunderstanding too. Without this fix, the machine
> > doesn't even boot. I'm not sure why perf is the blocking question here.
> 
> Well you can not degrade performance of existing machines that work
> today, right?  That would be a regression and it seems that you are
> doing that if I read the numbers above correctly.
> 

I agree that we shouldn't degrade performance of existing machines, but
this is a timing bug on existing platforms that have a slow enough cpu
such that emmc doesn't finish probing before dm-verity progresses to
trying to read off the device. In my opinion it's a bit unfair to trade
performance in the common case for not supporting all use cases. I'm
just trying to get my machines to boot without having to carry my own
patch here.

As a path forward I can add a command line flag as a bool to handle this
and that should hopefully take care of the regresion aspect to this.


> > Greg, is there another tree I can run this through?
> 
> Why would you want to route around a maintainer just to get a patch that
> would have to be reverted applied?  :)
> 

What's your advice here as I don't feel like I'm getting adequate
traction with Ulf. I think I've generally been quite patient here
waiting > 3 months for this patch to be reviewed.

Thanks,
Dennis