Message-ID: <CAJZ5v0jJEo5p4Wr_bZjHHOfQG4WomX9pFtBwFnU6eMJRoCctOA@mail.gmail.com>
Date: Mon, 4 Mar 2024 19:10:29 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Kai-Heng Feng <kai.heng.feng@...onical.com>, gregkh@...uxfoundation.org, 
	bhelgaas@...gle.com, linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Ricky Wu <ricky_wu@...ltek.com>, Kees Cook <keescook@...omium.org>, 
	Tony Luck <tony.luck@...el.com>, "Guilherme G. Piccoli" <gpiccoli@...lia.com>, 
	linux-hardening@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v3] driver core: Cancel scheduled pm_runtime_idle() on
 device removal

On Mon, Mar 4, 2024 at 6:00 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Mon, Mar 4, 2024 at 5:41 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Mon, Mar 4, 2024 at 4:51 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > >
> > > On Mon, Mar 04, 2024 at 03:38:38PM +0100, Rafael J. Wysocki wrote:
> > > > On Thu, Feb 29, 2024 at 7:23 AM Kai-Heng Feng
> > > > <kai.heng.feng@...onical.com> wrote:
> > > > >
> > > > > When inserting an SD7.0 card to Realtek card reader, the card reader
> > > > > unplugs itself and morph into a NVMe device. The slot Link down on hot
> > > > > unplugged can cause the following error:
> > > > >
> > > > > pcieport 0000:00:1c.0: pciehp: Slot(8): Link Down
> > > > > BUG: unable to handle page fault for address: ffffb24d403e5010
> > > > > PGD 100000067 P4D 100000067 PUD 1001fe067 PMD 100d97067 PTE 0
> > > > > Oops: 0000 [#1] PREEMPT SMP PTI
> > > > > CPU: 3 PID: 534 Comm: kworker/3:10 Not tainted 6.4.0 #6
> > > > > Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./H370M Pro4, BIOS P3.40 10/25/2018
> > > > > Workqueue: pm pm_runtime_work
> > > > > RIP: 0010:ioread32+0x2e/0x70
> > > > ...
> > > > > Call Trace:
> > > > >  <TASK>
> > > > >  ? show_regs+0x68/0x70
> > > > >  ? __die_body+0x20/0x70
> > > > >  ? __die+0x2b/0x40
> > > > >  ? page_fault_oops+0x160/0x480
> > > > >  ? search_bpf_extables+0x63/0x90
> > > > >  ? ioread32+0x2e/0x70
> > > > >  ? search_exception_tables+0x5f/0x70
> > > > >  ? kernelmode_fixup_or_oops+0xa2/0x120
> > > > >  ? __bad_area_nosemaphore+0x179/0x230
> > > > >  ? bad_area_nosemaphore+0x16/0x20
> > > > >  ? do_kern_addr_fault+0x8b/0xa0
> > > > >  ? exc_page_fault+0xe5/0x180
> > > > >  ? asm_exc_page_fault+0x27/0x30
> > > > >  ? ioread32+0x2e/0x70
> > > > >  ? rtsx_pci_write_register+0x5b/0x90 [rtsx_pci]
> > > > >  rtsx_set_l1off_sub+0x1c/0x30 [rtsx_pci]
> > > > >  rts5261_set_l1off_cfg_sub_d0+0x36/0x40 [rtsx_pci]
> > > > >  rtsx_pci_runtime_idle+0xc7/0x160 [rtsx_pci]
> > > > >  ? __pfx_pci_pm_runtime_idle+0x10/0x10
> > > > >  pci_pm_runtime_idle+0x34/0x70
> > > > >  rpm_idle+0xc4/0x2b0
> > > > >  pm_runtime_work+0x93/0xc0
> > > > >  process_one_work+0x21a/0x430
> > > > >  worker_thread+0x4a/0x3c0
> > > > ...
> > >
> > > > > This happens because scheduled pm_runtime_idle() is not cancelled.
> > > >
> > > > But rpm_resume() changes dev->power.request to RPM_REQ_NONE and if
> > > > pm_runtime_work() sees this, it will not run rpm_idle().
> > > >
> > > > However, rpm_resume() doesn't deactivate the autosuspend timer if it
> > > > is running (see the comment in rpm_resume() regarding this), so it may
> > > > queue up a runtime PM work later.
> > > >
> > > > If this is not desirable, you need to stop the autosuspend timer
> > > > explicitly in addition to calling pm_runtime_get_sync().
> > >
> > > I don't quite follow all this.  I think the race is between
> > > rtsx_pci_remove() (not resume) and rtsx_pci_runtime_idle().
> >
> > I think so too and the latter is not expected to run.
> >
> > >   rtsx_pci_remove()
> > >   {
> > >     pm_runtime_get_sync()
> > >     pm_runtime_forbid()
> > >     ...
> > >
> > > If this is an rtsx bug, what exactly should be added to
> > > rtsx_pci_remove()?
> > >
> > > Is there ever a case where we want any runtime PM work to happen
> > > during or after a driver .remove()?  If not, maybe the driver core
> > > should prevent that, which I think is basically what this patch does.
> >
> > No, it is not, because it doesn't actually prevent the race from
> > occurring, it just narrows the window quite a bit.
> >
> > It would be better to call pm_runtime_dont_use_autosuspend() instead
> > of pm_runtime_barrier().
> >
> > > If this is an rtsx driver bug, I'm concerned there may be many other
> > > drivers with a similar issue.  rtsx exercises this path more than most
> > > because the device switches between card reader and NVMe SSD using
> > > hotplug add/remove based on whether an SD card is inserted (see [1]).
> >
> > This is a valid concern, so it is mostly a matter of where to disable
> > autosuspend.
> >
> > It may be the driver core in principle, but note that it calls
> > ->remove() after invoking pm_runtime_put_sync(), so why would it
> > disable autosuspend when it allows runtime PM to race with device
> > removal in general?
> >
> > Another way might be to add a pm_runtime_dont_use_autosuspend() call
> > at the beginning of pci_device_remove().
> >
> > Or just remove the optimization in question from rpm_resume() which is
> > quite confusing and causes people to make assumptions that lead to
> > incorrect behavior in this particular case.
>
> Well, scratch this.
>
> If rpm_idle() is already running at the time rpm_resume() is called,
> the latter may return right away without waiting, which is incorrect.
>
> rpm_resume() needs to wait for the "idle" callback to complete, so
> this (again, modulo GMail-induced whitespace mangling) should help:
>
> ---
>  drivers/base/power/runtime.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> Index: linux-pm/drivers/base/power/runtime.c
> ===================================================================
> --- linux-pm.orig/drivers/base/power/runtime.c
> +++ linux-pm/drivers/base/power/runtime.c
> @@ -798,7 +798,8 @@ static int rpm_resume(struct device *dev
>      }
>
>      if (dev->power.runtime_status == RPM_RESUMING ||
> -        dev->power.runtime_status == RPM_SUSPENDING) {
> +        dev->power.runtime_status == RPM_SUSPENDING ||
> +        dev->power.idle_notification) {
>          DEFINE_WAIT(wait);
>
>          if (rpmflags & (RPM_ASYNC | RPM_NOWAIT)) {
> @@ -826,7 +827,8 @@ static int rpm_resume(struct device *dev
>              prepare_to_wait(&dev->power.wait_queue, &wait,
>                      TASK_UNINTERRUPTIBLE);
>              if (dev->power.runtime_status != RPM_RESUMING &&
> -                dev->power.runtime_status != RPM_SUSPENDING)
> +                dev->power.runtime_status != RPM_SUSPENDING &&
> +                !dev->power.idle_notification)
>                  break;
>
>              spin_unlock_irq(&dev->power.lock);

Well, not really.

The problem is that rtsx_pci_runtime_idle() is not expected to be
running after pm_runtime_get_sync(), but the latter doesn't really
guarantee that.  It only guarantees that the suspend/resume callbacks
will not be running after it returns.

As I said above, if the ->runtime_idle() callback is already running
when pm_runtime_get_sync() runs, the latter will notice that the
status is RPM_ACTIVE and will return right away without waiting for
the former to complete.  In fact, it cannot wait for it to complete,
because it may be called from a ->runtime_idle() callback itself (it
arguably does not make much sense to do that, but it is not strictly
forbidden).

So whoever is providing a ->runtime_idle() callback, they need to
protect it from running in parallel with whatever code runs after
pm_runtime_get_sync().  Note that ->runtime_idle() will not start
after pm_runtime_get_sync(), but it may continue running then if it
has started earlier already.

Calling pm_runtime_barrier() after pm_runtime_get_sync() (not before
it) should suffice, but once the runtime PM usage counter is dropped,
rpm_idle() may run again, so this is only effective until the usage
counter is greater than 1.  This means that
__device_release_driver(() is not the right place to call it, because
the usage counter is dropped before calling device_remove() in that
case.

The PCI bus type can prevent the race between driver-provided
->runtime_idle() and ->remove() from occurring by adding a
pm_runtime_probe() call in the following way:

---
 drivers/pci/pci-driver.c |    7 +++++++
 1 file changed, 7 insertions(+)

Index: linux-pm/drivers/pci/pci-driver.c
===================================================================
--- linux-pm.orig/drivers/pci/pci-driver.c
+++ linux-pm/drivers/pci/pci-driver.c
@@ -473,6 +473,13 @@ static void pci_device_remove(struct dev

     if (drv->remove) {
         pm_runtime_get_sync(dev);
+        /*
+         * If the driver provides a .runtime_idle() callback and it has
+         * started to run already, it may continue to run in parallel
+         * with the code below, so wait until all of the runtime PM
+         * activity has completed.
+         */
+        pm_runtime_barrier(dev);
         drv->remove(pci_dev);
         pm_runtime_put_noidle(dev);
     }

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