Re: [PATCH v2] drm/amdgpu: Replace Mutex with Spinlock for RLCG register access to avoid Priority Inversion in SRIOV

["Christian König" <christian.koenig@amd.com>]

Am 16.02.25 um 03:43 schrieb Srinivasan Shanmugam:
> RLCG Register Access is a way for virtual functions to safely access GPU
> registers in a virtualized environment., including TLB flushes and
> register reads. When multiple threads or VFs try to access the same
> registers simultaneously, it can lead to race conditions. By using the
> RLCG interface, the driver can serialize access to the registers. This
> means that only one thread can access the registers at a time,
> preventing conflicts and ensuring that operations are performed
> correctly. Additionally, when a low-priority task holds a mutex that a
> high-priority task needs, ie., If a thread holding a spinlock tries to
> acquire a mutex, it can lead to priority inversion. register access in
> amdgpu_virt_rlcg_reg_rw especially in a fast code path is critical.
>
> The call stack shows that the function amdgpu_virt_rlcg_reg_rw is being
> called, which attempts to acquire the mutex. This function is invoked
> from amdgpu_sriov_wreg, which in turn is called from
> gmc_v11_0_flush_gpu_tlb.
>
> The [ BUG: Invalid wait context ] indicates that a thread is trying to
> acquire a mutex while it is in a context that does not allow it to sleep
> (like holding a spinlock).
>
> Fixes the below:
>
> [  253.013423] =============================
> [  253.013434] [ BUG: Invalid wait context ]
> [  253.013446] 6.12.0-amdstaging-drm-next-lol-050225 #14 Tainted: G     U     OE
> [  253.013464] -----------------------------
> [  253.013475] kworker/0:1/10 is trying to lock:
> [  253.013487] ffff9f30542e3cf8 (&adev->virt.rlcg_reg_lock){+.+.}-{3:3}, at: amdgpu_virt_rlcg_reg_rw+0xf6/0x330 [amdgpu]
> [  253.013815] other info that might help us debug this:
> [  253.013827] context-{4:4}
> [  253.013835] 3 locks held by kworker/0:1/10:
> [  253.013847]  #0: ffff9f3040050f58 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x3f5/0x680
> [  253.013877]  #1: ffffb789c008be40 ((work_completion)(&wfc.work)){+.+.}-{0:0}, at: process_one_work+0x1d6/0x680
> [  253.013905]  #2: ffff9f3054281838 (&adev->gmc.invalidate_lock){+.+.}-{2:2}, at: gmc_v11_0_flush_gpu_tlb+0x198/0x4f0 [amdgpu]
> [  253.014154] stack backtrace:
> [  253.014164] CPU: 0 UID: 0 PID: 10 Comm: kworker/0:1 Tainted: G     U     OE      6.12.0-amdstaging-drm-next-lol-050225 #14
> [  253.014189] Tainted: [U]=USER, [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
> [  253.014203] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.1 11/18/2024
> [  253.014224] Workqueue: events work_for_cpu_fn
> [  253.014241] Call Trace:
> [  253.014250]  <TASK>
> [  253.014260]  dump_stack_lvl+0x9b/0xf0
> [  253.014275]  dump_stack+0x10/0x20
> [  253.014287]  __lock_acquire+0xa47/0x2810
> [  253.014303]  ? srso_alias_return_thunk+0x5/0xfbef5
> [  253.014321]  lock_acquire+0xd1/0x300
> [  253.014333]  ? amdgpu_virt_rlcg_reg_rw+0xf6/0x330 [amdgpu]
> [  253.014562]  ? __lock_acquire+0xa6b/0x2810
> [  253.014578]  __mutex_lock+0x85/0xe20
> [  253.014591]  ? amdgpu_virt_rlcg_reg_rw+0xf6/0x330 [amdgpu]
> [  253.014782]  ? sched_clock_noinstr+0x9/0x10
> [  253.014795]  ? srso_alias_return_thunk+0x5/0xfbef5
> [  253.014808]  ? local_clock_noinstr+0xe/0xc0
> [  253.014822]  ? amdgpu_virt_rlcg_reg_rw+0xf6/0x330 [amdgpu]
> [  253.015012]  ? srso_alias_return_thunk+0x5/0xfbef5
> [  253.015029]  mutex_lock_nested+0x1b/0x30
> [  253.015044]  ? mutex_lock_nested+0x1b/0x30
> [  253.015057]  amdgpu_virt_rlcg_reg_rw+0xf6/0x330 [amdgpu]
> [  253.015249]  amdgpu_sriov_wreg+0xc5/0xd0 [amdgpu]
> [  253.015435]  gmc_v11_0_flush_gpu_tlb+0x44b/0x4f0 [amdgpu]
> [  253.015667]  gfx_v11_0_hw_init+0x499/0x29c0 [amdgpu]
> [  253.015901]  ? __pfx_smu_v13_0_update_pcie_parameters+0x10/0x10 [amdgpu]
> [  253.016159]  ? srso_alias_return_thunk+0x5/0xfbef5
> [  253.016173]  ? smu_hw_init+0x18d/0x300 [amdgpu]
> [  253.016403]  amdgpu_device_init+0x29ad/0x36a0 [amdgpu]
> [  253.016614]  amdgpu_driver_load_kms+0x1a/0xc0 [amdgpu]
> [  253.017057]  amdgpu_pci_probe+0x1c2/0x660 [amdgpu]
> [  253.017493]  local_pci_probe+0x4b/0xb0
> [  253.017746]  work_for_cpu_fn+0x1a/0x30
> [  253.017995]  process_one_work+0x21e/0x680
> [  253.018248]  worker_thread+0x190/0x330
> [  253.018500]  ? __pfx_worker_thread+0x10/0x10
> [  253.018746]  kthread+0xe7/0x120
> [  253.018988]  ? __pfx_kthread+0x10/0x10
> [  253.019231]  ret_from_fork+0x3c/0x60
> [  253.019468]  ? __pfx_kthread+0x10/0x10
> [  253.019701]  ret_from_fork_asm+0x1a/0x30
> [  253.019939]  </TASK>
>
> Fixes: e864180ee49b ("drm/amdgpu: Add lock around VF RLCG interface")
> Cc: lin cao <lin.cao@amd.com>
> Cc: Jingwen Chen <Jingwen.Chen2@amd.com>
> Cc: Victor Skvortsov <victor.skvortsov@amd.com>
> Cc: Zhigang Luo <zhigang.luo@amd.com>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Alex Deucher <alexander.deucher@amd.com>
> Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>

Reviewed-by: Christian König <christian.koenig@amd.com>

> ---
> v2: s/spin_trylock/spin_lock_irqsave to be safe (Christian). 
>
>  drivers/gpu/drm/amd/amdgpu/amdgpu_device.c | 2 +-
>  drivers/gpu/drm/amd/amdgpu/amdgpu_virt.c   | 5 +++--
>  drivers/gpu/drm/amd/amdgpu/amdgpu_virt.h   | 3 ++-
>  3 files changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
> index eab530778fbd..14125cc3a937 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
> @@ -4251,7 +4251,6 @@ int amdgpu_device_init(struct amdgpu_device *adev,
>  	mutex_init(&adev->grbm_idx_mutex);
>  	mutex_init(&adev->mn_lock);
>  	mutex_init(&adev->virt.vf_errors.lock);
> -	mutex_init(&adev->virt.rlcg_reg_lock);
>  	hash_init(adev->mn_hash);
>  	mutex_init(&adev->psp.mutex);
>  	mutex_init(&adev->notifier_lock);
> @@ -4277,6 +4276,7 @@ int amdgpu_device_init(struct amdgpu_device *adev,
>  	spin_lock_init(&adev->se_cac_idx_lock);
>  	spin_lock_init(&adev->audio_endpt_idx_lock);
>  	spin_lock_init(&adev->mm_stats.lock);
> +	spin_lock_init(&adev->virt.rlcg_reg_lock);
>  	spin_lock_init(&adev->wb.lock);
>  
>  	xa_init_flags(&adev->userq_xa, XA_FLAGS_LOCK_IRQ);
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_virt.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_virt.c
> index 2056efaf157d..e6f0152e5b08 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_virt.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_virt.c
> @@ -1017,6 +1017,7 @@ u32 amdgpu_virt_rlcg_reg_rw(struct amdgpu_device *adev, u32 offset, u32 v, u32 f
>  	void *scratch_reg2;
>  	void *scratch_reg3;
>  	void *spare_int;
> +	unsigned long flags;
>  
>  	if (!adev->gfx.rlc.rlcg_reg_access_supported) {
>  		dev_err(adev->dev,
> @@ -1038,7 +1039,7 @@ u32 amdgpu_virt_rlcg_reg_rw(struct amdgpu_device *adev, u32 offset, u32 v, u32 f
>  	scratch_reg2 = (void __iomem *)adev->rmmio + 4 * reg_access_ctrl->scratch_reg2;
>  	scratch_reg3 = (void __iomem *)adev->rmmio + 4 * reg_access_ctrl->scratch_reg3;
>  
> -	mutex_lock(&adev->virt.rlcg_reg_lock);
> +	spin_lock_irqsave(&adev->virt.rlcg_reg_lock, flags);
>  
>  	if (reg_access_ctrl->spare_int)
>  		spare_int = (void __iomem *)adev->rmmio + 4 * reg_access_ctrl->spare_int;
> @@ -1097,7 +1098,7 @@ u32 amdgpu_virt_rlcg_reg_rw(struct amdgpu_device *adev, u32 offset, u32 v, u32 f
>  
>  	ret = readl(scratch_reg0);
>  
> -	mutex_unlock(&adev->virt.rlcg_reg_lock);
> +	spin_unlock_irqrestore(&adev->virt.rlcg_reg_lock, flags);
>  
>  	return ret;
>  }
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_virt.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_virt.h
> index 270a032e2d70..0f3ccae5c1ab 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_virt.h
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_virt.h
> @@ -279,7 +279,8 @@ struct amdgpu_virt {
>  	/* the ucode id to signal the autoload */
>  	uint32_t autoload_ucode_id;
>  
> -	struct mutex rlcg_reg_lock;
> +	/* Spinlock to protect access to the RLCG register interface */
> +	spinlock_t rlcg_reg_lock;
>  
>  	union amd_sriov_ras_caps ras_en_caps;
>  	union amd_sriov_ras_caps ras_telemetry_en_caps;