Re: [PATCH 2/2] PM: domain: use per-genpd lockdep class

[Ulf Hansson <ulf.hansson@linaro.org>]

+ Mark

On Fri, 11 Jun 2021 at 16:34, Dmitry Baryshkov
<dmitry.baryshkov@linaro.org> wrote:
>
> Added Stephen to Cc list
>
> On Fri, 11 Jun 2021 at 16:50, Ulf Hansson <ulf.hansson@linaro.org> wrote:
> >
> > On Fri, 11 Jun 2021 at 12:15, Dmitry Baryshkov
> > <dmitry.baryshkov@linaro.org> wrote:
> > >
> > > In case of nested genpds it is easy to get the following warning from
> > > lockdep, because all genpd's mutexes share same locking class. Use the
> > > per-genpd locking class to stop lockdep from warning about possible
> > > deadlocks. It is not possible to directly use genpd nested locking, as
> > > it is not the genpd code calling genpd. There are interim calls to
> > > regulator core.
> > >
> > > [    3.030219] ============================================
> > > [    3.030220] WARNING: possible recursive locking detected
> > > [    3.030221] 5.13.0-rc3-00054-gf8f0a2f2b643-dirty #2480 Not tainted
> > > [    3.030222] --------------------------------------------
> > > [    3.030223] kworker/u16:0/7 is trying to acquire lock:
> > > [    3.030224] ffffde0eabd29aa0 (&genpd->mlock){+.+.}-{3:3}, at: genpd_lock_mtx+0x18/0x2c
> > > [    3.030236]
> > > [    3.030236] but task is already holding lock:
> > > [    3.030236] ffffde0eabcfd6d0 (&genpd->mlock){+.+.}-{3:3}, at: genpd_lock_mtx+0x18/0x2c
> > > [    3.030240]
> > > [    3.030240] other info that might help us debug this:
> > > [    3.030240]  Possible unsafe locking scenario:
> > > [    3.030240]
> > > [    3.030241]        CPU0
> > > [    3.030241]        ----
> > > [    3.030242]   lock(&genpd->mlock);
> > > [    3.030243]   lock(&genpd->mlock);
> > > [    3.030244]
> > > [    3.030244]  *** DEADLOCK ***
> > > [    3.030244]
> > > [    3.030244]  May be due to missing lock nesting notation
> > > [    3.030244]
> > > [    3.030245] 6 locks held by kworker/u16:0/7:
> > > [    3.030246]  #0: ffff6cca00010938 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x1f0/0x730
> > > [    3.030252]  #1: ffff8000100c3db0 (deferred_probe_work){+.+.}-{0:0}, at: process_one_work+0x1f0/0x730
> > > [    3.030255]  #2: ffff6cca00ce3188 (&dev->mutex){....}-{3:3}, at: __device_attach+0x3c/0x184
> > > [    3.030260]  #3: ffffde0eabcfd6d0 (&genpd->mlock){+.+.}-{3:3}, at: genpd_lock_mtx+0x18/0x2c
> > > [    3.030264]  #4: ffff8000100c3968 (regulator_ww_class_acquire){+.+.}-{0:0}, at: regulator_lock_dependent+0x6c/0x1b0
> > > [    3.030270]  #5: ffff6cca00a59158 (regulator_ww_class_mutex){+.+.}-{3:3}, at: regulator_lock_recursive+0x94/0x1d0
> > > [    3.030273]
> > > [    3.030273] stack backtrace:
> > > [    3.030275] CPU: 6 PID: 7 Comm: kworker/u16:0 Not tainted 5.13.0-rc3-00054-gf8f0a2f2b643-dirty #2480
> > > [    3.030276] Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
> > > [    3.030278] Workqueue: events_unbound deferred_probe_work_func
> > > [    3.030280] Call trace:
> > > [    3.030281]  dump_backtrace+0x0/0x1a0
> > > [    3.030284]  show_stack+0x18/0x24
> > > [    3.030286]  dump_stack+0x108/0x188
> > > [    3.030289]  __lock_acquire+0xa20/0x1e0c
> > > [    3.030292]  lock_acquire.part.0+0xc8/0x320
> > > [    3.030294]  lock_acquire+0x68/0x84
> > > [    3.030296]  __mutex_lock+0xa0/0x4f0
> > > [    3.030299]  mutex_lock_nested+0x40/0x50
> > > [    3.030301]  genpd_lock_mtx+0x18/0x2c
> > > [    3.030303]  dev_pm_genpd_set_performance_state+0x94/0x1a0
> > > [    3.030305]  reg_domain_enable+0x28/0x4c
> > > [    3.030308]  _regulator_do_enable+0x420/0x6b0
> > > [    3.030310]  _regulator_enable+0x178/0x1f0
> > > [    3.030312]  regulator_enable+0x3c/0x80
> >
> > At a closer look, I am pretty sure that it's the wrong code design
> > that triggers this problem, rather than that we have a real problem in
> > genpd. To put it simply, the code in genpd isn't designed to work like
> > this. We will end up in circular looking paths, leading to deadlocks,
> > sooner or later if we allow the above code path.
> >
> > To fix it, the regulator here needs to be converted to a proper PM
> > domain. This PM domain should be assigned as the parent to the one
> > that is requested to be powered on.
>
> This more or less resembles original design, replaced per review
> request to use separate regulator
> (https://lore.kernel.org/linux-arm-msm/160269659638.884498.4031967462806977493@swboyd.mtv.corp.google.com/,
> https://lore.kernel.org/linux-arm-msm/20201023131925.334864-1-dmitry.baryshkov@linaro.org/).

Thanks for the pointers. In hindsight, it looks like the
"regulator-fixed-domain" DT binding wasn't the right thing.

Fortunately, it looks like the problem can be quite easily fixed, by
moving to a correct model of the domain hierarchy.

Beyond this, perhaps we should consider removing the
"regulator-fixed-domain" DT property, as to avoid similar problems
from cropping up?

Mark, what do you think?

>
> Stephen, would it be fine to you to convert the mmcx regulator into
> the PM domain?
>
> > > [    3.030314]  gdsc_toggle_logic+0x30/0x124
> > > [    3.030317]  gdsc_enable+0x60/0x290
> > > [    3.030318]  _genpd_power_on+0xc0/0x134
> > > [    3.030320]  genpd_power_on.part.0+0xa4/0x1f0
> > > [    3.030322]  __genpd_dev_pm_attach+0xf4/0x1b0
> > > [    3.030324]  genpd_dev_pm_attach+0x60/0x70
> > > [    3.030326]  dev_pm_domain_attach+0x54/0x5c
> > > [    3.030329]  platform_probe+0x50/0xe0
> > > [    3.030330]  really_probe+0xe4/0x510
> > > [    3.030332]  driver_probe_device+0x64/0xcc
> > > [    3.030333]  __device_attach_driver+0xb8/0x114
> > > [    3.030334]  bus_for_each_drv+0x78/0xd0
> > > [    3.030337]  __device_attach+0xdc/0x184
> > > [    3.030338]  device_initial_probe+0x14/0x20
> > > [    3.030339]  bus_probe_device+0x9c/0xa4
> > > [    3.030340]  deferred_probe_work_func+0x88/0xc4
> > > [    3.030342]  process_one_work+0x298/0x730
> > > [    3.030343]  worker_thread+0x74/0x470
> > > [    3.030344]  kthread+0x168/0x170
> > > [    3.030346]  ret_from_fork+0x10/0x34
> > >
> > > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

[...]

Kind regards
Uffe