linux-kernel - Re: [PATCH] mmc: don't request CD IRQ until mmc_start

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Date:	Wed, 17 Sep 2014 13:57:42 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Ulf Hansson <ulf.hansson@...aro.org>
CC:	Chris Ball <chris@...ntf.net>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stephen Warren <swarren@...dia.com>,
	Russell King <linux@....linux.org.uk>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Alexandre Courbot <acourbot@...dia.com>,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH] mmc: don't request CD IRQ until mmc_start_host()

On 09/17/2014 01:55 PM, Ulf Hansson wrote:
> On 12 September 2014 19:18, Stephen Warren <swarren@...dotorg.org> wrote:
>> From: Stephen Warren <swarren@...dia.com>
>>
>> As soon as the CD IRQ is requested, it can trigger, since it's an
>> externally controlled event. If it does, delayed_work host->detect will
>> be scheduled.
>>
>> Many host controller probe()s are roughly structured as:
>>
>> *_probe() {
>>      host = sdhci_pltfm_init();
>>      mmc_of_parse(host->mmc);
>>      rc = sdhci_add_host(host);
>>      if (rc) {
>>          sdhci_pltfm_free();
>>          return rc;
>>      }
>>
>> In 3.17, CD IRQs can are enabled quite early via *_probe() ->
>> mmc_of_parse() -> mmc_gpio_request_cd() -> mmc_gpiod_request_cd_irq().
>>
>> Note that in linux-next, mmc_of_parse() calls mmc_gpio*d*_request_cd()
>> rather than mmc_gpio_request_cd(), and mmc_gpio*d*_request_cd() doesn't
>> call mmc_gpiod_request_cd_irq(). However, this issue still exists for
>> any other direct users of mmc_gpio_request_cd().
>>
>> sdhci_add_host() may fail part way through (e.g. due to deferred
>> probe for a vmmc regulator), and sdhci_pltfm_free() does nothing to
>> unrequest the CD IRQ nor cancel the delayed_work. sdhci_pltfm_free() is
>> coded to assume that if sdhci_add_host() failed, then the delayed_work
>> cannot (or should not) have been triggered.
>>
>> This can lead to the following with CONFIG_DEBUG_OBJECTS_* enabled, when
>> kfree(host) is eventually called inside sdhci_pltfm_free():
>>
>> WARNING: CPU: 2 PID: 6 at lib/debugobjects.c:263 debug_print_object+0x8c/0xb4()
>> ODEBUG: free active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x18
>>
>> The object being complained about is host->detect.
>>
>> There's no need to request the CD IRQ so early; mmc_start_host() already
>> requests it, and I *assume* that mmc_start_host() is called somehow for
>> all host controllers. For SDHCI hosts at least, the typical call path
>> that does this is: *_probe() -> sdhci_add_host() -> mmc_add_host() ->
>> mmc_start_host(). So, remove the call to mmc_gpiod_request_cd_irq() from
>> mmc_gpio_request_cd(). This matches mmc_gpio*d*_request_cd(), which
>> already doesn't call mmc_gpiod_request_cd_irq().
>>
>> This solves the problem (eliminates the kernel error message above),
>> since it guarantees that the IRQ can't trigger before mmc_start_host()
>> is called.
>>
>> The critical point here is that once sdhci_add_host() calls
>> mmc_add_host() -> mmc_start_host(), sdhci_add_host() is coded not to
>> fail. In other words, if there's a chance that mmc_start_host() may have
>> been called, and CD IRQs triggered, and the delayed_work scheduled,
>> sdhci_add_host() won't fail, and so cleanup is no longer via
>> sdhci_pltfm_free() (which doesn't free the IRQ or cancel the work queue)
>> but instead must be via sdhci_remove_host(), which calls mmc_remove_host()
>> -> mmc_stop_host(), which does free the IRQ and cancel the work queue.
>>
>> This fixes what I might conclude to be a mistake in commit 740a221ef0e5
>> ("mmc: slot-gpio: Add GPIO descriptor based CD GPIO API"), which added the
>> call from mmc_start_host() to mmc_gpiod_request_cd_irq(), but also added
>> incorrectly added a call from mmc_gpio_request_cd() to
>> mmc_gpiod_request_cd_irq().
>>
>> CC: Russell King <linux@....linux.org.uk>
>> Cc: Adrian Hunter <adrian.hunter@...el.com>
>> Cc: Alexandre Courbot <acourbot@...dia.com>
>> Cc: Linus Walleij <linus.walleij@...aro.org>
>> Signed-off-by: Stephen Warren <swarren@...dia.com>
>
> Hi Stephen,
>
> Thanks for looking into this. It seems like this issue has been
> present for quite a while.
> I believe your patch should have a stable tag for 3.15+ as well,
> unless you object I will add it.

Yes, that probably makes sense, thanks.
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