linux-kernel - Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

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Date:   Tue, 4 Jul 2017 15:08:43 +0200
From:   Mason <slash.tmp@...e.fr>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

On 04/07/2017 09:09, Peter Zijlstra wrote:

> On Mon, Jul 03, 2017 at 05:30:28PM +0200, Marc Gonzalez wrote:
> 
>> And at the end of smp8759_config_read:
>>
>> 	printk("in_atomic_preempt_off = %d\n", in_atomic_preempt_off());
> 
> That's confused...

That much is certain. I am indeed grasping at straws.

I grepped "scheduling while atomic", found __schedule_bug()
in kernel/sched/core.c and saw

	if (IS_ENABLED(CONFIG_DEBUG_PREEMPT) && in_atomic_preempt_off()) {
		pr_err("Preemption disabled at:");
		print_ip_sym(preempt_disable_ip);
		pr_cont("\n");
	}

I thought printing the value of in_atomic_preempt_off()
in the callback would indicate whether preemption had
already been turned off at that point.

It doesn't work like that?

BTW, why didn't print_ip_sym(preempt_disable_ip); say
where preemption had been disabled?

>> [    1.026568] BUG: scheduling while atomic: swapper/0/1/0x00000002
>> [    1.032625] 5 locks held by swapper/0/1:
>> [    1.036575]  #0:  (&dev->mutex){......}, at: [<c038c684>] __driver_attach+0x50/0xd0
>> [    1.044319]  #1:  (&dev->mutex){......}, at: [<c038c694>] __driver_attach+0x60/0xd0
>> [    1.052050]  #2:  (pci_lock){+.+...}, at: [<c03309d8>] pci_bus_read_config_dword+0x44/0x94
> 
> This is a raw_spinlock_t, that disables preemption

drivers/pci/access.c

/*
 * This interrupt-safe spinlock protects all accesses to PCI
 * configuration space.
 */
DEFINE_RAW_SPINLOCK(pci_lock);

	raw_spin_lock_irqsave(&pci_lock, flags);
	res = bus->ops->read(bus, devfn, pos, len, &data);

IIUC, it's not possible to call stop_machine() while holding
a raw spinlock? What about regular spinlocks? IIUC, in RT,
regular spinlocks may sleep?

I didn't find "preempt" or "schedul" in the spinlock doc.
https://www.kernel.org/doc/Documentation/locking/spinlocks.txt

> Using stop_machine() is per definition doing it wrong ;-)

Here's the high-level view. My HW is borked and muxes
config space and mem space. So I need a way to freeze
the entire system, make the config space access, and
then return the system to normal. (AFAICT, config space
accesses are rare, so if I kill performance for these
accesses, the system might remain usable.)

Is there a way to do this? Mark suggested stop_machine
but it seems using it in my situation is not quite
straight-forward.

Regards.