linux-kernel - Re: [PATCH] pci_get_device call from interrupt in reboot fixups

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Date:	Tue, 07 Aug 2007 11:48:42 +0400
From:	"Denis V. Lunev" <den@...ru>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Greg KH <gregkh@...e.de>, "Denis V. Lunev" <den@...nvz.org>,
	dev@...nvz.org, devel@...nvz.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pci_get_device call from interrupt in reboot fixups

Andrew Morton wrote:
> On Mon, 6 Aug 2007 19:49:10 -0700 Greg KH <gregkh@...e.de> wrote:
> 
>> On Mon, Aug 06, 2007 at 11:16:20AM +0400, Denis V. Lunev wrote:
>>> Greg KH wrote:
>>>> On Fri, Aug 03, 2007 at 02:39:24PM +0400, Denis V. Lunev wrote:
>>>>> The following calltrace is possible now:
>>>>>  handle_sysrq
>>>>>    machine_emergency_restart
>>>>>      mach_reboot_fixups
>>>>>        pci_get_device
>>>>>          pci_get_subsys
>>>>> 	   down_read
>>>>> The patch obtains PCI device during initialization to avoid bothering PCI
>>>>> search engine in interrupt. Devices used in this code are not supposed to
>>>>> be pluggable, so it looks safe to keep them.
>>>> What devices are supposed to be affected here?  Are you sure that they
>>>> can't be removed later?  Grabbing references here might mess with them
>>>> in the future.
>>> Right now the list is the following:
>>> static struct device_fixup fixups_table[] = {
>>> { PCI_VENDOR_ID_CYRIX, PCI_DEVICE_ID_CYRIX_5530_LEGACY,
>>> cs5530a_warm_reset },
>>> { PCI_VENDOR_ID_AMD, PCI_DEVICE_ID_AMD_CS5536_ISA, cs5536_warm_reset },
>>> };
>>>
>>> Though, if the approach is not suitable, we can skip fixups if we came
>>> from sysrq.
>> I don't think we really need to do fixups when we are "crashing" like
>> this.  The user really isn't shutting down the kernel as it should
>> normally do.
>>
>> Andrew, I really don't want to change the PCI core to handle this, as we
>> finally fixed a lot of issues with drivers trying to walk these lists
>> from interrupt context.  So if you want to just hide the warning message
>> as we are shutting down, that's fine with me.  Or just don't do the
>> fixups.  But grabbing a reference to the pci device is unsafe in my
>> opinion and I do not want to do that.
>>
> 
> OK, good decision ;)
> 
> One approach would be for some brave soul to pick his way through
> the reboot code and ensure that we are correctly and reliably setting
> system_state to SYSTEM_RESTART, then test that in __might_sleep().
> 
> But this does suppress somewhat-useful debugging just because of sysrq-B
> and I really wouldn't want to utilise the horrid system_state any more that
> we are presently doing.  I think on balance that it would be better if we
> could do something more targetted, like modify emergency_restart() to test
> in_interrupt() and to then apologetically set some well-named global flag
> which will shut up __might_sleep().  Pretty foul, but I can't think of
> anything better.

__might_sleep prevention will solve the problem only partially :( There
is a direct WARN_ON(in_interrupt()) in pci_get_subsys.

IMHO, calling down_read(&pci_bus_sem); from sysrq-B is not an option.
I'll send a fixup disabling patch in a moment.
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