[PATCH v2] drm/i915/hwmon: Fix locking inversion in sysfs getter

[Janusz Krzysztofik]

Hi Ashutosh,

On Tuesday, 12 March 2024 17:25:14 CET Dixit, Ashutosh wrote:
> On Mon, 11 Mar 2024 13:34:58 -0700, Janusz Krzysztofik wrote:
> >
> > In i915 hwmon sysfs getter path we now take a hwmon_lock, then acquire an
> > rpm wakeref.  That results in lock inversion:
> >
> > <4> [197.079335] ======================================================
> > <4> [197.085473] WARNING: possible circular locking dependency detected
> > <4> [197.091611] 6.8.0-rc7-Patchwork_129026v7-gc4dc92fb1152+ #1 Not tainted
> > <4> [197.098096] ------------------------------------------------------
> > <4> [197.104231] prometheus-node/839 is trying to acquire lock:
> > <4> [197.109680] ffffffff82764d80 (fs_reclaim){+.+.}-{0:0}, at: __kmalloc+0x9a/0x350
> > <4> [197.116939]
> > but task is already holding lock:
> > <4> [197.122730] ffff88811b772a40 (&hwmon->hwmon_lock){+.+.}-{3:3}, at: hwm_energy+0x4b/0x100 [i915]
> > <4> [197.131543]
> > which lock already depends on the new lock.
> > ...
> > <4> [197.507922] Chain exists of:
> >   fs_reclaim --> &gt->reset.mutex --> &hwmon->hwmon_lock
> > <4> [197.518528]  Possible unsafe locking scenario:
> > <4> [197.524411]        CPU0                    CPU1
> > <4> [197.528916]        ----                    ----
> > <4> [197.533418]   lock(&hwmon->hwmon_lock);
> > <4> [197.537237]                                lock(&gt->reset.mutex);
> > <4> [197.543376]                                lock(&hwmon->hwmon_lock);
> > <4> [197.549682]   lock(fs_reclaim);
> > ...
> > <4> [197.632548] Call Trace:
> > <4> [197.634990]  <TASK>
> > <4> [197.637088]  dump_stack_lvl+0x64/0xb0
> > <4> [197.640738]  check_noncircular+0x15e/0x180
> > <4> [197.652968]  check_prev_add+0xe9/0xce0
> > <4> [197.656705]  __lock_acquire+0x179f/0x2300
> > <4> [197.660694]  lock_acquire+0xd8/0x2d0
> > <4> [197.673009]  fs_reclaim_acquire+0xa1/0xd0
> > <4> [197.680478]  __kmalloc+0x9a/0x350
> > <4> [197.689063]  acpi_ns_internalize_name.part.0+0x4a/0xb0
> > <4> [197.694170]  acpi_ns_get_node_unlocked+0x60/0xf0
> > <4> [197.720608]  acpi_ns_get_node+0x3b/0x60
> > <4> [197.724428]  acpi_get_handle+0x57/0xb0
> > <4> [197.728164]  acpi_has_method+0x20/0x50
> > <4> [197.731896]  acpi_pci_set_power_state+0x43/0x120
> > <4> [197.736485]  pci_power_up+0x24/0x1c0
> > <4> [197.740047]  pci_pm_default_resume_early+0x9/0x30
> > <4> [197.744725]  pci_pm_runtime_resume+0x2d/0x90
> > <4> [197.753911]  __rpm_callback+0x3c/0x110
> > <4> [197.762586]  rpm_callback+0x58/0x70
> > <4> [197.766064]  rpm_resume+0x51e/0x730
> > <4> [197.769542]  rpm_resume+0x267/0x730
> > <4> [197.773020]  rpm_resume+0x267/0x730
> > <4> [197.776498]  rpm_resume+0x267/0x730
> > <4> [197.779974]  __pm_runtime_resume+0x49/0x90
> > <4> [197.784055]  __intel_runtime_pm_get+0x19/0xa0 [i915]
> > <4> [197.789070]  hwm_energy+0x55/0x100 [i915]
> > <4> [197.793183]  hwm_read+0x9a/0x310 [i915]
> > <4> [197.797124]  hwmon_attr_show+0x36/0x120
> > <4> [197.800946]  dev_attr_show+0x15/0x60
> > <4> [197.804509]  sysfs_kf_seq_show+0xb5/0x100
> >
> > Acquire the wakeref before the lock and hold it as long as the lock is
> > also held.  Follow that pattern across the whole source file where similar
> > lock inversion can happen.
> >
> > v2: Keep hardware read under the lock so the whole operation of updating
> >     energy from hardware is still atomic (Guenter),
> >   - instead, acquire the rpm wakeref before the lock and hold it as long
> >     as the lock is held,
> >   - use the same aproach for other similar places across the i915_hwmon.c
> >     source file (Rodrigo).
> >
> > Fixes: c41b8bdcc297 ("drm/i915/hwmon: Show device level energy usage")
> 
> I would think that the lock inversion issue was introduced here:
> 
> 1b44019a93e2 ("drm/i915/guc: Disable PL1 power limit when loading GuC firmware")
> 
> This is the commit which introduced this sequence:
> 	lock(&gt->reset.mutex);
> 	lock(&hwmon->hwmon_lock);
> 
> Before this, everything was fine. So perhaps the Fixes tag should reference
> this commit?

OK, thanks for pointing that out.

> Otherwise the patch LGTM:
> 
> Reviewed-by: Ashutosh Dixit <ashutosh.dixit at intel.com>

Thank you,
Janusz