[PATCH] pci: Fix a possible sleep-in-atomic bug in pci_set_power_state

[Bjorn Helgaas]

On Mon, Oct 09, 2017 at 12:15:17PM -0500, Bjorn Helgaas wrote:
> [+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?

Sigh, could have saved myself some time if I'd read the whole thread
before responding :)  Sorry for repeating what Greg already said!

> > Signed-off-by: Jia-Ju Bai <baijiaju1990 at 163.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
> > 
> >