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Subject: Re: [PATCH v2 4/5] clk: Get runtime PM before walking tree during disable_unused
From: Stephen Boyd <sboyd@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-clk@vger.kernel.org, patches@lists.linux.dev, linux-arm-msm@vger.kernel.org, Douglas Anderson <dianders@chromium.org>, Marek Szyprowski <m.szyprowski@samsung.com>, Ulf Hansson <ulf.hansson@linaro.org>, Krzysztof Kozlowski <krzk@kernel.org>
To: Michael Turquette <mturquette@baylibre.com>, Stephen Boyd <sboyd@kernel.org>
Date: Sun, 07 Apr 2024 19:36:49 -0700
User-Agent: alot/0.10

Quoting Stephen Boyd (2024-03-25 11:41:58)
> Doug reported [1] the following hung task:
>=20
>  INFO: task swapper/0:1 blocked for more than 122 seconds.
>        Not tainted 5.15.149-21875-gf795ebc40eb8 #1
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>  task:swapper/0       state:D stack:    0 pid:    1 ppid:     0 flags:0x0=
0000008
>  Call trace:
>   __switch_to+0xf4/0x1f4
>   __schedule+0x418/0xb80
>   schedule+0x5c/0x10c
>   rpm_resume+0xe0/0x52c
>   rpm_resume+0x178/0x52c
>   __pm_runtime_resume+0x58/0x98
>   clk_pm_runtime_get+0x30/0xb0
>   clk_disable_unused_subtree+0x58/0x208
>   clk_disable_unused_subtree+0x38/0x208
>   clk_disable_unused_subtree+0x38/0x208
>   clk_disable_unused_subtree+0x38/0x208
>   clk_disable_unused_subtree+0x38/0x208
>   clk_disable_unused+0x4c/0xe4
>   do_one_initcall+0xcc/0x2d8
>   do_initcall_level+0xa4/0x148
>   do_initcalls+0x5c/0x9c
>   do_basic_setup+0x24/0x30
>   kernel_init_freeable+0xec/0x164
>   kernel_init+0x28/0x120
>   ret_from_fork+0x10/0x20
>  INFO: task kworker/u16:0:9 blocked for more than 122 seconds.
>        Not tainted 5.15.149-21875-gf795ebc40eb8 #1
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>  task:kworker/u16:0   state:D stack:    0 pid:    9 ppid:     2 flags:0x0=
0000008
>  Workqueue: events_unbound deferred_probe_work_func
>  Call trace:
>   __switch_to+0xf4/0x1f4
>   __schedule+0x418/0xb80
>   schedule+0x5c/0x10c
>   schedule_preempt_disabled+0x2c/0x48
>   __mutex_lock+0x238/0x488
>   __mutex_lock_slowpath+0x1c/0x28
>   mutex_lock+0x50/0x74
>   clk_prepare_lock+0x7c/0x9c
>   clk_core_prepare_lock+0x20/0x44
>   clk_prepare+0x24/0x30
>   clk_bulk_prepare+0x40/0xb0
>   mdss_runtime_resume+0x54/0x1c8
>   pm_generic_runtime_resume+0x30/0x44
>   __genpd_runtime_resume+0x68/0x7c
>   genpd_runtime_resume+0x108/0x1f4
>   __rpm_callback+0x84/0x144
>   rpm_callback+0x30/0x88
>   rpm_resume+0x1f4/0x52c
>   rpm_resume+0x178/0x52c
>   __pm_runtime_resume+0x58/0x98
>   __device_attach+0xe0/0x170
>   device_initial_probe+0x1c/0x28
>   bus_probe_device+0x3c/0x9c
>   device_add+0x644/0x814
>   mipi_dsi_device_register_full+0xe4/0x170
>   devm_mipi_dsi_device_register_full+0x28/0x70
>   ti_sn_bridge_probe+0x1dc/0x2c0
>   auxiliary_bus_probe+0x4c/0x94
>   really_probe+0xcc/0x2c8
>   __driver_probe_device+0xa8/0x130
>   driver_probe_device+0x48/0x110
>   __device_attach_driver+0xa4/0xcc
>   bus_for_each_drv+0x8c/0xd8
>   __device_attach+0xf8/0x170
>   device_initial_probe+0x1c/0x28
>   bus_probe_device+0x3c/0x9c
>   deferred_probe_work_func+0x9c/0xd8
>   process_one_work+0x148/0x518
>   worker_thread+0x138/0x350
>   kthread+0x138/0x1e0
>   ret_from_fork+0x10/0x20
>=20
> The first thread is walking the clk tree and calling
> clk_pm_runtime_get() to power on devices required to read the clk
> hardware via struct clk_ops::is_enabled(). This thread holds the clk
> prepare_lock, and is trying to runtime PM resume a device, when it finds
> that the device is in the process of resuming so the thread schedule()s
> away waiting for the device to finish resuming before continuing. The
> second thread is runtime PM resuming the same device, but the runtime
> resume callback is calling clk_prepare(), trying to grab the
> prepare_lock waiting on the first thread.
>=20
> This is a classic ABBA deadlock. To properly fix the deadlock, we must
> never runtime PM resume or suspend a device with the clk prepare_lock
> held. Actually doing that is near impossible today because the global
> prepare_lock would have to be dropped in the middle of the tree, the
> device runtime PM resumed/suspended, and then the prepare_lock grabbed
> again to ensure consistency of the clk tree topology. If anything
> changes with the clk tree in the meantime, we've lost and will need to
> start the operation all over again.
>=20
> Luckily, most of the time we're simply incrementing or decrementing the
> runtime PM count on an active device, so we don't have the chance to
> schedule away with the prepare_lock held. Let's fix this immediate
> problem that can be triggered more easily by simply booting on Qualcomm
> sc7180.
>=20
> Introduce a list of clk_core structures that have been registered, or
> are in the process of being registered, that require runtime PM to
> operate. Iterate this list and call clk_pm_runtime_get() on each of them
> without holding the prepare_lock during clk_disable_unused(). This way
> we can be certain that the runtime PM state of the devices will be
> active and resumed so we can't schedule away while walking the clk tree
> with the prepare_lock held. Similarly, call clk_pm_runtime_put() without
> the prepare_lock held to properly drop the runtime PM reference. We
> remove the calls to clk_pm_runtime_{get,put}() in this path because
> they're superfluous now that we know the devices are runtime resumed.
>=20
> Reported-by: Douglas Anderson <dianders@chromium.org>
> Closes: https://lore.kernel.org/all/20220922084322.RFC.2.I375b6b9e0a0a534=
8962f004beb3dafee6a12dfbb@changeid/ [1]
> Closes: https://issuetracker.google.com/328070191
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Ulf Hansson <ulf.hansson@linaro.org>
> Cc: Krzysztof Kozlowski <krzk@kernel.org>
> Fixes: 9a34b45397e5 ("clk: Add support for runtime PM")
> Signed-off-by: Stephen Boyd <sboyd@kernel.org>
> ---

Applied to clk-fixes