In the Linux kernel, the following vulnerability has been resolved:clk: Get runtime PM before walking tree during disable_unusedDoug reported [1] the following hung task: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:0x00000008
 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:0x00000008
 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+0x10/0x20The 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.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.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
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