On Mon, Mar 11, 2024 at 09:06:46AM +0100, 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 --> >->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(>->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
>
> However, the lock is only intended to protect either a hwmon overflow
> counter or rmw hardware operations. There is no need to hold the lock,
> only the wakeref, while reading from hardware.
>
> Acquire the lock after hardware read under rpm wakeref.
>
> Fixes: c41b8bdcc297 ("drm/i915/hwmon: Show device level energy usage")
> Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@xxxxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v6.2+
> ---
> drivers/gpu/drm/i915/i915_hwmon.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c b/drivers/gpu/drm/i915/i915_hwmon.c
> index 8c3f443c8347e..faf7670de6e06 100644
> --- a/drivers/gpu/drm/i915/i915_hwmon.c
> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> @@ -136,11 +136,11 @@ hwm_energy(struct hwm_drvdata *ddat, long *energy)
> else
> rgaddr = hwmon->rg.energy_status_all;
>
> - mutex_lock(&hwmon->hwmon_lock);
> -
> with_intel_runtime_pm(uncore->rpm, wakeref)
> reg_val = intel_uncore_read(uncore, rgaddr);
>
> + mutex_lock(&hwmon->hwmon_lock);
> +
This is not enough.
check hwm_locked_with_pm_intel_uncore_rmw()
It looks like we need to rethink this lock entirely here.
struct mutex hwmon_lock; /* counter overflow \
logic and rmw */
do we really need to protect the rmw?
This also shows me that we have other places with intel_uncore_rmw
without the with runtime_pm.
perhaps we need to stop using the with_rpm macro and use explicit
rpm calls before the hwmon_locks before the rmw?
perhaps we need a more granular lock?
notice that I'm just brainstorming/thinking out loud... someone need
to go there an run an deeper analisys on this lock and rpm calls
on hwmon.
> if (reg_val >= ei->reg_val_prev)
> ei->accum_energy += reg_val - ei->reg_val_prev;
> else
> --
> 2.43.0
>
