linux-kernel - Re: [PATCH] pci: Fix a possible sleep-in-atomic bug in pci_set_power

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Message-ID: <20171009171517.GL25517@bhelgaas-glaptop.roam.corp.google.com>
Date:   Mon, 9 Oct 2017 12:15:17 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jia-Ju Bai <baijiaju1990@....com>
Cc:     patrik.r.jakobsson@...il.com, airlied@...ux.ie,
        bhelgaas@...gle.com, forest@...ttletooquiet.net,
        gregkh@...uxfoundation.org, simon@...anor.nu, scott@...heina.com,
        tvboxspy@...il.com, dan.a.cashman@...il.com,
        golubev.mikhail@...il.com, devel@...verdev.osuosl.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org
Subject: Re: [PATCH] pci: Fix a possible sleep-in-atomic bug in
 pci_set_power_state

[+cc Rafael, linux-pm]

Hi Jia-Ju,

On Mon, Oct 09, 2017 at 04:16:20PM +0800, Jia-Ju Bai wrote:
> The drivers vt6655 and gma500 call pci_set_power_state under a spinlock, which may sleep.
> The function call paths are:
> gma_power_begin (acquire the spinlock) (drivers/gpu/drm/gma500/power.c)
>   gma_resume_pci
>     pci_set_power_state
>       __pci_start_power_transition (drivers/pci/pci.c)
>         msleep --> may sleep
> 
> gma_power_begin (acquire the spinlock) (drivers/gpu/drm/gma500/power.c)
>   gma_resume_pci
>     pci_enable_device
>       pci_enable_device_flags (drivers/pci/pci.c)
>         do_pci_enable_device
>           pci_set_power_state
>             __pci_start_power_transition
>               msleep --> may sleep
> 
> vt6655_suspend (acquire the spinlock) (drivers/staging/vt6655/device_main.c)
>   pci_set_power_state
>     __pci_start_power_transition (drivers/pci/pci.c)
>       msleep --> may sleep
> 
> To fix these bugs, msleep is replaced with mdelay in __pci_start_power_transition
> 
> These bugs are found by my static analysis tool and my code review.

We can either

  - change pci_set_power_state() so it can be called while holding a
    spinlock (as this patch does), or
    
  - change the drivers so they don't hold the spinlock while calling 
    pci_set_power_state().

I think the latter is better because d3cold_delay is typically 100ms,
and that's a long time to spin with interrupts disabled.

I assume it's easy to produce an actual failure here?  Why haven't we
seen bug reports about this?

Bjorn

> Signed-off-by: Jia-Ju Bai <baijiaju1990@....com>
> ---
>  drivers/pci/pci.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 6078dfc..7b763a3 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -823,7 +823,7 @@ static void __pci_start_power_transition(struct pci_dev *dev, pci_power_t state)
>  		 */
>  		if (dev->runtime_d3cold) {
>  			if (dev->d3cold_delay)
> -				msleep(dev->d3cold_delay);
> +				mdelay(dev->d3cold_delay);
>  			/*
>  			 * When powering on a bridge from D3cold, the
>  			 * whole hierarchy may be powered on into
> -- 
> 1.7.9.5
> 
>