Date:   Tue, 24 Mar 2020 16:36:56 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
        Mark Brown <broonie@...nel.org>,
        Felipe Balbi <balbi@...nel.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Ferry Toth <fntoth@...il.com>
Subject: Re: [PATCH v2 1/3] driver core: Break infinite loop when deferred
 probe can't be satisfied

 On Tue, Mar 24, 2020 at 2:39 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Tue, Mar 24, 2020 at 01:52:00PM +0100, Rafael J. Wysocki wrote:
> > On Tue, Mar 24, 2020 at 1:20 PM Andy Shevchenko
> > <andriy.shevchenko@...ux.intel.com> wrote:
> > >
> > > Consider the following scenario.
> > >
> > > The main driver of USB OTG controller (dwc3-pci), which has the following
> > > functional dependencies on certain platform:
> > > - ULPI (tusb1210)
> > > - extcon (tested with extcon-intel-mrfld)
> > >
> > > Note, that first driver, tusb1210, is available at the moment of
> > > dwc3-pci probing, while extcon-intel-mrfld is built as a module and
> > > won't appear till user space does something about it.
> > >
> > > This is depicted by kernel configuration excerpt:
> > >
> > >         CONFIG_PHY_TUSB1210=y
> > >         CONFIG_USB_DWC3=y
> > >         CONFIG_USB_DWC3_ULPI=y
> > >         CONFIG_USB_DWC3_DUAL_ROLE=y
> > >         CONFIG_USB_DWC3_PCI=y
> > >         CONFIG_EXTCON_INTEL_MRFLD=m
> > >
> > > In the Buildroot environment the modules are probed by alphabetical ordering
> > > of their modaliases. The latter comes to the case when USB OTG driver will be
> > > probed first followed by extcon one.
> > >
> > > So, if the platform anticipates extcon device to be appeared, in the above case
> > > we will get deferred probe of USB OTG, because of ordering.
> > >
> > > Since current implementation, done by the commit 58b116bce136 ("drivercore:
> > > deferral race condition fix") counts the amount of triggered deferred probe,
> > > we never advance the situation -- the change makes it to be an infinite loop.
> > >
> > > ---8<---8<---
> > >
> > > [   22.187127] driver_deferred_probe_trigger <<< 1
> > >
> > > ...here is the late initcall triggers deferred probe...
> > >
> > > [   22.191725] platform dwc3.0.auto: deferred_probe_work_func in deferred list
> > >
> > > ...dwc3.0.auto is the only device in the deferred list...
> > >
> > > [   22.198727] platform dwc3.0.auto: deferred_probe_work_func 1 <<< counter 1
> > >
> > > ...the counter before mutex is unlocked is kept the same...
> > >
> > > [   22.205663] platform dwc3.0.auto: Retrying from deferred list
> > >
> > > ...mutes has been unlocked, we try to re-probe the driver...
> > >
> > > [   22.211487] bus: 'platform': driver_probe_device: matched device dwc3.0.auto with driver dwc3
> > > [   22.220060] bus: 'platform': really_probe: probing driver dwc3 with device dwc3.0.auto
> > > [   22.238735] bus: 'ulpi': driver_probe_device: matched device dwc3.0.auto.ulpi with driver tusb1210
> > > [   22.247743] bus: 'ulpi': really_probe: probing driver tusb1210 with device dwc3.0.auto.ulpi
> > > [   22.256292] driver: 'tusb1210': driver_bound: bound to device 'dwc3.0.auto.ulpi'
> > > [   22.263723] driver_deferred_probe_trigger <<< 2
> > >
> > > ...the dwc3.0.auto probes ULPI, we got successful bound and bumped counter...
> > >
> > > [   22.268304] bus: 'ulpi': really_probe: bound device dwc3.0.auto.ulpi to driver tusb1210
> > > [   22.276697] platform dwc3.0.auto: Driver dwc3 requests probe deferral
> > >
> > > ...but extcon driver is still missing...
> > >
> > > [   22.283174] platform dwc3.0.auto: Added to deferred list
> > > [   22.288513] platform dwc3.0.auto: driver_deferred_probe_add_trigger local counter: 1 new counter 2
> > >
> > > ...and since we had a successful probe, we got counter mismatch...
> > >
> > > [   22.297490] driver_deferred_probe_trigger <<< 3
> > > [   22.302074] platform dwc3.0.auto: deferred_probe_work_func 2 <<< counter 3
> > >
> > > ...at the end we have a new counter and loop repeats again, see 22.198727...
> > >
> > > ---8<---8<---
> > >
> > > Revert of the commit helps, but it is probably not helpful for the initially
> > > found regression. Artem Bityutskiy suggested to use counter of the successful
> > > probes instead. This fixes above mentioned case and shouldn't prevent driver
> > > to reprobe deferred ones.
> > >
> > > Fixes: 58b116bce136 ("drivercore: deferral race condition fix")
> > > Suggested-by: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> > > Cc: Grant Likely grant.likely@....com
> > > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > Cc: Mark Brown <broonie@...nel.org>
> > > Cc: Felipe Balbi <balbi@...nel.org>
> > > Cc: Andrzej Hajda <a.hajda@...sung.com>
> > > Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > > Reviewed-by: Peter Ujfalusi <peter.ujfalusi@...com>
> > > Tested-by: Ferry Toth <fntoth@...il.com>
> > > ---
> > > v2: picked up tags, update Grant's email (Peter)
> > >  drivers/base/dd.c | 39 +++++++++++++++++++++------------------
> > >  1 file changed, 21 insertions(+), 18 deletions(-)
> > >
> > > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > > index b25bcab2a26b..43720beb5300 100644
> > > --- a/drivers/base/dd.c
> > > +++ b/drivers/base/dd.c
> > > @@ -53,7 +53,6 @@
> > >  static DEFINE_MUTEX(deferred_probe_mutex);
> > >  static LIST_HEAD(deferred_probe_pending_list);
> > >  static LIST_HEAD(deferred_probe_active_list);
> > > -static atomic_t deferred_trigger_count = ATOMIC_INIT(0);
> > >  static struct dentry *deferred_devices;
> > >  static bool initcalls_done;
> > >
> > > @@ -147,17 +146,6 @@ static bool driver_deferred_probe_enable = false;
> > >   * This functions moves all devices from the pending list to the active
> > >   * list and schedules the deferred probe workqueue to process them.  It
> > >   * should be called anytime a driver is successfully bound to a device.
> > > - *
> > > - * Note, there is a race condition in multi-threaded probe. In the case where
> > > - * more than one device is probing at the same time, it is possible for one
> > > - * probe to complete successfully while another is about to defer. If the second
> > > - * depends on the first, then it will get put on the pending list after the
> > > - * trigger event has already occurred and will be stuck there.
> > > - *
> > > - * The atomic 'deferred_trigger_count' is used to determine if a successful
> > > - * trigger has occurred in the midst of probing a driver. If the trigger count
> > > - * changes in the midst of a probe, then deferred processing should be triggered
> > > - * again.
> > >   */
> > >  static void driver_deferred_probe_trigger(void)
> > >  {
> > > @@ -170,7 +158,6 @@ static void driver_deferred_probe_trigger(void)
> > >          * into the active list so they can be retried by the workqueue
> > >          */
> > >         mutex_lock(&deferred_probe_mutex);
> > > -       atomic_inc(&deferred_trigger_count);
> > >         list_splice_tail_init(&deferred_probe_pending_list,
> > >                               &deferred_probe_active_list);
> > >         mutex_unlock(&deferred_probe_mutex);
> > > @@ -350,6 +337,19 @@ static void __exit deferred_probe_exit(void)
> > >  }
> > >  __exitcall(deferred_probe_exit);
> > >
> > > +/*
> > > + * Note, there is a race condition in multi-threaded probe. In the case where
> > > + * more than one device is probing at the same time, it is possible for one
> > > + * probe to complete successfully while another is about to defer. If the second
> > > + * depends on the first, then it will get put on the pending list after the
> > > + * trigger event has already occurred and will be stuck there.
> > > + *
> > > + * The atomic 'probe_okay' is used to determine if a successful probe has
> > > + * occurred in the midst of probing another driver. If the count changes in
> > > + * the midst of a probe, then deferred processing should be triggered again.
> > > + */
> > > +static atomic_t probe_okay = ATOMIC_INIT(0);
> > > +
> > >  /**
> > >   * device_is_bound() - Check if device is bound to a driver
> > >   * @dev: device to check
> > > @@ -375,6 +375,7 @@ static void driver_bound(struct device *dev)
> > >         pr_debug("driver: '%s': %s: bound to device '%s'\n", dev->driver->name,
> > >                  __func__, dev_name(dev));
> > >
> > > +       atomic_inc(&probe_okay);
> > >         klist_add_tail(&dev->p->knode_driver, &dev->driver->p->klist_devices);
> > >         device_links_driver_bound(dev);
> > >
> > > @@ -481,18 +482,18 @@ static atomic_t probe_count = ATOMIC_INIT(0);
> > >  static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue);
> > >
> > >  static void driver_deferred_probe_add_trigger(struct device *dev,
> > > -                                             int local_trigger_count)
> > > +                                             int local_probe_okay_count)
> > >  {
> > >         driver_deferred_probe_add(dev);
> > >         /* Did a trigger occur while probing? Need to re-trigger if yes */
> > > -       if (local_trigger_count != atomic_read(&deferred_trigger_count))
> > > +       if (local_probe_okay_count != atomic_read(&probe_okay))
> > >                 driver_deferred_probe_trigger();
> > >  }
> > >
> > >  static int really_probe(struct device *dev, struct device_driver *drv)
> > >  {
> > >         int ret = -EPROBE_DEFER;
> > > -       int local_trigger_count = atomic_read(&deferred_trigger_count);
> > > +       int local_probe_okay_count = atomic_read(&probe_okay);
> > >         bool test_remove = IS_ENABLED(CONFIG_DEBUG_TEST_DRIVER_REMOVE) &&
> > >                            !drv->suppress_bind_attrs;
> > >
> > > @@ -509,7 +510,7 @@ static int really_probe(struct device *dev, struct device_driver *drv)
> > >
> > >         ret = device_links_check_suppliers(dev);
> > >         if (ret == -EPROBE_DEFER)
> > > -               driver_deferred_probe_add_trigger(dev, local_trigger_count);
> > > +               driver_deferred_probe_add_trigger(dev, local_probe_okay_count);
> > >         if (ret)
> > >                 return ret;
> > >
> > > @@ -619,7 +620,7 @@ static int really_probe(struct device *dev, struct device_driver *drv)
> > >         case -EPROBE_DEFER:
> > >                 /* Driver requested deferred probing */
> > >                 dev_dbg(dev, "Driver %s requests probe deferral\n", drv->name);
> > > -               driver_deferred_probe_add_trigger(dev, local_trigger_count);
> > > +               driver_deferred_probe_add_trigger(dev, local_probe_okay_count);
> > >                 break;
> > >         case -ENODEV:
> > >         case -ENXIO:
> > > @@ -1148,6 +1149,8 @@ static void __device_release_driver(struct device *dev, struct device *parent)
> > >                 dev_pm_set_driver_flags(dev, 0);
> > >
> > >                 klist_remove(&dev->p->knode_driver);
> > > +               atomic_dec(&probe_okay);
> > > +
> >
> > Why is this needed?
>
> Under successful probe the following is comprehended. When probe of the driver
> happens it may be discarded (as in above case) as it was initiated by another
> driver which got deferred.
>
> We also discussed this with Peter [1] during his review.
>
> [1]: https://lkml.org/lkml/2020/3/12/347

OK, but I would add a comment explaining that to the code.

Also it would be good to explain why probe_okay cannot go below zero
here in the changelog.

Cheers!

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