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Date:   Wed, 18 May 2022 12:50:02 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     Tanjore Suresh <tansuresh@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Christoph Hellwig <hch@lst.de>,
        Sagi Grimberg <sagi@grimberg.me>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-nvme <linux-nvme@lists.infradead.org>,
        Linux PCI <linux-pci@vger.kernel.org>
Subject: Re: [PATCH v3 1/3] driver core: Support asynchronous driver shutdown
Message-ID: <20220518175002.GA1148748@bhelgaas>
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In-Reply-To: <CAJZ5v0ibBw41YSfSWF1CtY7w9oLO+8bKNK2AK0grE0qabJ6QQA@mail.gmail.com>
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On Wed, May 18, 2022 at 01:38:49PM +0200, Rafael J. Wysocki wrote:
> On Wed, May 18, 2022 at 12:08 AM Tanjore Suresh <tansuresh@google.com> wrote:
> >
> > This changes the bus driver interface with additional entry points
> > to enable devices to implement asynchronous shutdown. The existing
> > synchronous interface to shutdown is unmodified and retained for
> > backward compatibility.
> >
> > This changes the common device shutdown code to enable devices to
> > participate in asynchronous shutdown implementation.
> >
> > Signed-off-by: Tanjore Suresh <tansuresh@google.com>
> > ---
> >  drivers/base/core.c        | 38 +++++++++++++++++++++++++++++++++++++-
> >  include/linux/device/bus.h | 12 ++++++++++++
> >  2 files changed, 49 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index 3d6430eb0c6a..ba267ae70a22 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -4479,6 +4479,7 @@ EXPORT_SYMBOL_GPL(device_change_owner);
> >  void device_shutdown(void)
> >  {
> >         struct device *dev, *parent;
> > +       LIST_HEAD(async_shutdown_list);
> >
> >         wait_for_device_probe();
> >         device_block_probing();
> > @@ -4523,7 +4524,13 @@ void device_shutdown(void)
> >                                 dev_info(dev, "shutdown_pre\n");
> >                         dev->class->shutdown_pre(dev);
> >                 }
> > -               if (dev->bus && dev->bus->shutdown) {
> > +               if (dev->bus && dev->bus->async_shutdown_start) {
> > +                       if (initcall_debug)
> > +                               dev_info(dev, "async_shutdown_start\n");
> > +                       dev->bus->async_shutdown_start(dev);
> > +                       list_add_tail(&dev->kobj.entry,
> > +                               &async_shutdown_list);
> > +               } else if (dev->bus && dev->bus->shutdown) {
> >                         if (initcall_debug)
> >                                 dev_info(dev, "shutdown\n");
> >                         dev->bus->shutdown(dev);
> > @@ -4543,6 +4550,35 @@ void device_shutdown(void)
> >                 spin_lock(&devices_kset->list_lock);
> >         }
> >         spin_unlock(&devices_kset->list_lock);
> > +
> > +       /*
> > +        * Second pass spin for only devices, that have configured
> > +        * Asynchronous shutdown.
> > +        */
> > +       while (!list_empty(&async_shutdown_list)) {
> > +               dev = list_entry(async_shutdown_list.next, struct device,
> > +                               kobj.entry);
> > +               parent = get_device(dev->parent);
> > +               get_device(dev);
> > +               /*
> > +                * Make sure the device is off the  list
> > +                */
> > +               list_del_init(&dev->kobj.entry);
> > +               if (parent)
> > +                       device_lock(parent);
> > +               device_lock(dev);
> > +               if (dev->bus && dev->bus->async_shutdown_end) {
> > +                       if (initcall_debug)
> > +                               dev_info(dev,
> > +                               "async_shutdown_end called\n");
> > +                       dev->bus->async_shutdown_end(dev);
> > +               }
> > +               device_unlock(dev);
> > +               if (parent)
> > +                       device_unlock(parent);
> > +               put_device(dev);
> > +               put_device(parent);
> > +       }
> >  }
> >
> >  /*
> > diff --git a/include/linux/device/bus.h b/include/linux/device/bus.h
> > index a039ab809753..f582c9d21515 100644
> > --- a/include/linux/device/bus.h
> > +++ b/include/linux/device/bus.h
> > @@ -49,6 +49,16 @@ struct fwnode_handle;
> >   *             will never get called until they do.
> >   * @remove:    Called when a device removed from this bus.
> >   * @shutdown:  Called at shut-down time to quiesce the device.
> > + * @async_shutdown_start:      Called at the shutdown-time to start
> > + *                             the shutdown process on the device.
> > + *                             This entry point will be called only
> > + *                             when the bus driver has indicated it would
> > + *                             like to participate in asynchronous shutdown
> > + *                             completion.
> > + * @async_shutdown_end:        Called at shutdown-time  to complete the shutdown
> > + *                     process of the device. This entry point will be called
> > + *                     only when the bus drive has indicated it would like to
> > + *                     participate in the asynchronous shutdown completion.
> 
> I'm going to repeat my point here, but only once.
> 
> I see no reason to do async shutdown this way, instead of adding a
> flag for drivers to opt in for calling their existing shutdown
> callbacks asynchronously, in analogy with the async suspend and resume
> implementation.

There's a lot of code here that mere mortals like myself don't
understand very well, so here's my meager understanding of how
async suspend works and what you're suggesting to make this a
little more concrete.

Devices have this async_suspend bit:

  struct device {
    struct dev_pm_info {
      unsigned int async_suspend:1;

Drivers call device_enable_async_suspend() to set async_suspend if
they want it.  The system suspend path is something like this:

  suspend_enter
    dpm_suspend_noirq(PMSG_SUSPEND)
      dpm_noirq_suspend_devices(PMSG_SUSPEND)
        pm_transition = PMSG_SUSPEND
        while (!list_empty(&dpm_late_early_list))
          device_suspend_noirq(dev)
            dpm_async_fn(dev, async_suspend_noirq)
              if (is_async(dev))
                async_schedule_dev(async_suspend_noirq)       # async path

                  async_suspend_noirq               # called asynchronously
                  __device_suspend_noirq(dev, PMSG_SUSPEND, true)
                    callback = pm_noirq_op(PMSG_SUSPEND) # .suspend_noirq()
                    dpm_run_callback(callback)      # async call

            __device_suspend_noirq(dev, pm_transition, false) # sync path
              callback = pm_noirq_op(PMSG_SUSPEND)  # .suspend_noirq()
              dpm_run_callback(callback)            # sync call

        async_synchronize_full                                # wait

If a driver has called device_enable_async_suspend(), we'll use the
async_schedule_dev() path to schedule the appropriate .suspend_noirq()
method.  After scheduling it via the async path or directly calling it
via the sync path, the async_synchronize_full() waits for completion
of all the async methods.

I assume your suggestion is to do something like this:

  struct device {
    struct dev_pm_info {
      unsigned int async_suspend:1;
 +    unsigned int async_shutdown:1;

 + void device_enable_async_shutdown(struct device *dev)
 +   dev->power.async_shutdown = true;

    device_shutdown
      while (!list_empty(&devices_kset->list))
 -      dev->...->shutdown()
 +      if (is_async_shutdown(dev))
 +        async_schedule_dev(async_shutdown)   # async path
 +
 +         async_shutdown               # called asynchronously
 +           dev->...->shutdown()
 +
 +      else
 +        dev->...->shutdown()                 # sync path
 +
 +    async_synchronize_full                   # wait