linux-kernel - Re: [PATCH] PCI/PME: Fix race on PME polling

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Date:   Tue, 18 Jun 2019 12:45:31 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Keith Busch <keith.busch@...el.com>,
        Lukas Wunner <lukas@...ner.de>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Alex Williamson <alex.williamson@...hat.com>,
        Alexandru Gagniuc <mr.nuke.me@...il.com>
Subject: Re: [PATCH] PCI/PME: Fix race on PME polling

On Tue, Jun 18, 2019 at 12:41:01AM +0200, Rafael J. Wysocki wrote:
> On Monday, June 17, 2019 4:35:10 PM CEST Mika Westerberg wrote:
> > On Mon, Jun 17, 2019 at 12:37:06PM +0200, Rafael J. Wysocki wrote:
> > > On Sunday, June 9, 2019 1:29:33 PM CEST Lukas Wunner wrote:
> > > > Since commit df17e62e5bff ("PCI: Add support for polling PME state on
> > > > suspended legacy PCI devices"), the work item pci_pme_list_scan() polls
> > > > the PME status flag of devices and wakes them up if the bit is set.
> > > > 
> > > > The function performs a check whether a device's upstream bridge is in
> > > > D0 for otherwise the device is inaccessible, rendering PME polling
> > > > impossible.  However the check is racy because it is performed before
> > > > polling the device.  If the upstream bridge runtime suspends to D3hot
> > > > after pci_pme_list_scan() checks its power state and before it invokes
> > > > pci_pme_wakeup(), the latter will read the PMCSR as "all ones" and
> > > > mistake it for a set PME status flag.  I am seeing this race play out as
> > > > a Thunderbolt controller going to D3cold and occasionally immediately
> > > > going to D0 again because PM polling was performed at just the wrong
> > > > time.
> > > > 
> > > > Avoid by checking for an "all ones" PMCSR in pci_check_pme_status().
> > > > 
> > > > Fixes: 58ff463396ad ("PCI PM: Add function for checking PME status of devices")
> > > > Tested-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > > > Signed-off-by: Lukas Wunner <lukas@...ner.de>
> > > > Cc: stable@...r.kernel.org # v2.6.34+
> > > > Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > > ---
> > > >  drivers/pci/pci.c | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > index 8abc843b1615..eed5db9f152f 100644
> > > > --- a/drivers/pci/pci.c
> > > > +++ b/drivers/pci/pci.c
> > > > @@ -1989,6 +1989,8 @@ bool pci_check_pme_status(struct pci_dev *dev)
> > > >  	pci_read_config_word(dev, pmcsr_pos, &pmcsr);
> > > >  	if (!(pmcsr & PCI_PM_CTRL_PME_STATUS))
> > > >  		return false;
> > > > +	if (pmcsr == 0xffff)
> > > > +		return false;
> > > >  
> > > >  	/* Clear PME status. */
> > > >  	pmcsr |= PCI_PM_CTRL_PME_STATUS;
> > > > 
> > > 
> > > Added to my 5.3 queue, thanks!
> > 
> > Today when doing some PM testing I noticed that this patch actually
> > reveals an issue in our native PME handling. Problem is in
> > pcie_pme_handle_request() where we first convert req_id to struct
> > pci_dev and then call pci_check_pme_status() for it. Now, when a device
> > triggers wake the link is first brought up and then the PME is sent to
> > root complex with req_id matching the originating device. However, if
> > there are PCIe ports in the middle they may still be in D3 which means
> > that pci_check_pme_status() returns 0xffff for the device below so there
> > are lots of
> > 
> > 	Spurious native interrupt"
> > 
> > messages in the dmesg but the actual PME is never handled.
> > 
> > It has been working because pci_check_pme_status() returned true in case
> > of 0xffff as well and we went and runtime resumed to originating device.
> 
> In this case 0xffff is as good as PME Status set, that is the device needs to be
> resumed.
> 
> This is a regression in the $subject patch, not a bug in the PME code.

OK, thanks for explanation.

> > I think the correct way to handle this is actually drop the call to
> > pci_check_pme_status() in pcie_pme_handle_request() because the whole
> > idea of req_id in PME message is to allow the root complex and SW to
> > identify the device without need to poll for the PME status bit.
> 
> Not really, because if there is a PCIe-to-PCI bridge below the port, it is
> expected to use the req_id of the bridge for all of the devices below it.

Right, I forgot about that so indeed we need to check for the PME status
in that case to find out the correct device.