Re: [PATCH v4] drm/i915: Avoid circular locking dependency when flush delayed work on gt reset

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Hi Zhanjun,

On Thu, Jul 27, 2023 at 01:13:23PM -0700, Zhanjun Dong wrote:
> This attempts to avoid circular locking dependency between flush delayed work and intel_gt_reset.
> Switched from cancel_delayed_work_sync to cancel_delayed_work, the non-sync version for reset path, it is safe as the worker has the trylock code to handle the lock; Meanwhile keep the sync version for park/fini to ensure the worker is not still running during suspend or shutdown.

Next time, please wrap the sentences to 65 characters (standing
to the e-mail netiquette, RFC1855[1]) or 70-75 characters
(standing to the kernel guidelines[2]).

[1] https://www.ietf.org/rfc/rfc1855.txt
    chapter "2.1.1 For mail", page 3
[2] https://docs.kernel.org/process/submitting-patches.html
    chapter "The canonical patch format"

> WARNING: possible circular locking dependency detected
> 6.4.0-rc1-drmtip_1340-g31e3463b0edb+ #1 Not tainted
> ------------------------------------------------------
> kms_pipe_crc_ba/6415 is trying to acquire lock:
> ffff88813e6cc640 ((work_completion)(&(&guc->timestamp.work)->work)){+.+.}-{0:0}, at: __flush_work+0x42/0x530
> 
> but task is already holding lock:
> ffff88813e6cce90 (&gt->reset.mutex){+.+.}-{3:3}, at: intel_gt_reset+0x19e/0x470 [i915]
> 
> which lock already depends on the new lock.
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #3 (&gt->reset.mutex){+.+.}-{3:3}:
>         lock_acquire+0xd8/0x2d0
>         i915_gem_shrinker_taints_mutex+0x31/0x50 [i915]
>         intel_gt_init_reset+0x65/0x80 [i915]
>         intel_gt_common_init_early+0xe1/0x170 [i915]
>         intel_root_gt_init_early+0x48/0x60 [i915]
>         i915_driver_probe+0x671/0xcb0 [i915]
>         i915_pci_probe+0xdc/0x210 [i915]
>         pci_device_probe+0x95/0x120
>         really_probe+0x164/0x3c0
>         __driver_probe_device+0x73/0x160
>         driver_probe_device+0x19/0xa0
>         __driver_attach+0xb6/0x180
>         bus_for_each_dev+0x77/0xd0
>         bus_add_driver+0x114/0x210
>         driver_register+0x5b/0x110
>         __pfx_vgem_open+0x3/0x10 [vgem]
>         do_one_initcall+0x57/0x270
>         do_init_module+0x5f/0x220
>         load_module+0x1ca4/0x1f00
>         __do_sys_finit_module+0xb4/0x130
>         do_syscall_64+0x3c/0x90
>         entry_SYSCALL_64_after_hwframe+0x72/0xdc
> 
> -> #2 (fs_reclaim){+.+.}-{0:0}:
>         lock_acquire+0xd8/0x2d0
>         fs_reclaim_acquire+0xac/0xe0
>         kmem_cache_alloc+0x32/0x260
>         i915_vma_instance+0xb2/0xc60 [i915]
>         i915_gem_object_ggtt_pin_ww+0x175/0x370 [i915]
>         vm_fault_gtt+0x22d/0xf60 [i915]
>         __do_fault+0x2f/0x1d0
>         do_pte_missing+0x4a/0xd20
>         __handle_mm_fault+0x5b0/0x790
>         handle_mm_fault+0xa2/0x230
>         do_user_addr_fault+0x3ea/0xa10
>         exc_page_fault+0x68/0x1a0
>         asm_exc_page_fault+0x26/0x30
> 
> -> #1 (&gt->reset.backoff_srcu){++++}-{0:0}:
>         lock_acquire+0xd8/0x2d0
>         _intel_gt_reset_lock+0x57/0x330 [i915]
>         guc_timestamp_ping+0x35/0x130 [i915]
>         process_one_work+0x250/0x510
>         worker_thread+0x4f/0x3a0
>         kthread+0xff/0x130
>         ret_from_fork+0x29/0x50
> 
> -> #0 ((work_completion)(&(&guc->timestamp.work)->work)){+.+.}-{0:0}:
>         check_prev_add+0x90/0xc60
>         __lock_acquire+0x1998/0x2590
>         lock_acquire+0xd8/0x2d0
>         __flush_work+0x74/0x530
>         __cancel_work_timer+0x14f/0x1f0
>         intel_guc_submission_reset_prepare+0x81/0x4b0 [i915]
>         intel_uc_reset_prepare+0x9c/0x120 [i915]
>         reset_prepare+0x21/0x60 [i915]
>         intel_gt_reset+0x1dd/0x470 [i915]
>         intel_gt_reset_global+0xfb/0x170 [i915]
>         intel_gt_handle_error+0x368/0x420 [i915]
>         intel_gt_debugfs_reset_store+0x5c/0xc0 [i915]
>         i915_wedged_set+0x29/0x40 [i915]
>         simple_attr_write_xsigned.constprop.0+0xb4/0x110
>         full_proxy_write+0x52/0x80
>         vfs_write+0xc5/0x4f0
>         ksys_write+0x64/0xe0
>         do_syscall_64+0x3c/0x90
>         entry_SYSCALL_64_after_hwframe+0x72/0xdc
> 
> other info that might help us debug this:
>  Chain exists of:
>   (work_completion)(&(&guc->timestamp.work)->work) --> fs_reclaim --> &gt->reset.mutex
>   Possible unsafe locking scenario:
>         CPU0                    CPU1
>         ----                    ----
>    lock(&gt->reset.mutex);
>                                 lock(fs_reclaim);
>                                 lock(&gt->reset.mutex);
>    lock((work_completion)(&(&guc->timestamp.work)->work));
> 
>  *** DEADLOCK ***
>  3 locks held by kms_pipe_crc_ba/6415:
>   #0: ffff888101541430 (sb_writers#15){.+.+}-{0:0}, at: ksys_write+0x64/0xe0
>   #1: ffff888136c7eab8 (&attr->mutex){+.+.}-{3:3}, at: simple_attr_write_xsigned.constprop.0+0x47/0x110
>   #2: ffff88813e6cce90 (&gt->reset.mutex){+.+.}-{3:3}, at: intel_gt_reset+0x19e/0x470 [i915]
> 
> v2: Add sync flag to guc_cancel_busyness_worker to ensure reset path calls asynchronous cancel.
> v3: Add sync flag to intel_guc_submission_disable to ensure reset path calls asynchronous cancel.
> v4: Set to always sync from __uc_fini_hw path.

Thanks for taking care of this, there was a period we could see
this splatter everywhere :)

> Signed-off-by: Zhanjun Dong <zhanjun.dong@xxxxxxxxx>
> Cc: John Harrison <John.C.Harrison@xxxxxxxxx>
> Cc: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>
> ---
>  .../gpu/drm/i915/gt/uc/intel_guc_submission.c   | 17 ++++++++++-------
>  .../gpu/drm/i915/gt/uc/intel_guc_submission.h   |  2 +-
>  drivers/gpu/drm/i915/gt/uc/intel_uc.c           |  4 ++--
>  3 files changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> index a0e3ef1c65d2..ef4300246ce1 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> @@ -1357,9 +1357,12 @@ static void guc_enable_busyness_worker(struct intel_guc *guc)
>  	mod_delayed_work(system_highpri_wq, &guc->timestamp.work, guc->timestamp.ping_delay);
>  }
>  
> -static void guc_cancel_busyness_worker(struct intel_guc *guc)
> +static void guc_cancel_busyness_worker(struct intel_guc *guc, bool sync)
>  {
> -	cancel_delayed_work_sync(&guc->timestamp.work);
> +	if (sync)
> +		cancel_delayed_work_sync(&guc->timestamp.work);
> +	else
> +		cancel_delayed_work(&guc->timestamp.work);

The guc_cancel_busyness_worker() wrapper is made to make life
simpler, in oder not to force the caller to find
guc->timestamp.work. But if we add a true/false value, then we
make it again difficult because we need to go and check what they
mean, so that we decrease readability.

I would rather prefer something like:

	static void guc_cancel_busyness_worker_sync(struct intel_guc *guc)
	{
		cancel_delayed_work_sync(&guc->timestamp.work);
	}

	static void guc_cancel_busyness_worker(struct intel_guc *guc)
	{
		cancel_delayed_work(&guc->timestamp.work);
	}

We could perhaps improve this with defines or inlines, but I like
this way more.

What do you think?

Andi

>  }
>  
>  static void __reset_guc_busyness_stats(struct intel_guc *guc)
> @@ -1370,7 +1373,7 @@ static void __reset_guc_busyness_stats(struct intel_guc *guc)
>  	unsigned long flags;
>  	ktime_t unused;
>  
> -	guc_cancel_busyness_worker(guc);
> +	guc_cancel_busyness_worker(guc, false);
>  
>  	spin_lock_irqsave(&guc->timestamp.lock, flags);
>  
> @@ -1485,7 +1488,7 @@ static int guc_init_engine_stats(struct intel_guc *guc)
>  
>  static void guc_fini_engine_stats(struct intel_guc *guc)
>  {
> -	guc_cancel_busyness_worker(guc);
> +	guc_cancel_busyness_worker(guc, true);
>  }
>  
>  void intel_guc_busyness_park(struct intel_gt *gt)
> @@ -1500,7 +1503,7 @@ void intel_guc_busyness_park(struct intel_gt *gt)
>  	 * and causes an unclaimed register access warning. Cancel the worker
>  	 * synchronously here.
>  	 */
> -	guc_cancel_busyness_worker(guc);
> +	guc_cancel_busyness_worker(guc, true);
>  
>  	/*
>  	 * Before parking, we should sample engine busyness stats if we need to.
> @@ -4501,9 +4504,9 @@ int intel_guc_submission_enable(struct intel_guc *guc)
>  }
>  
>  /* Note: By the time we're here, GuC may have already been reset */
> -void intel_guc_submission_disable(struct intel_guc *guc)
> +void intel_guc_submission_disable(struct intel_guc *guc, bool sync)
>  {
> -	guc_cancel_busyness_worker(guc);
> +	guc_cancel_busyness_worker(guc, sync);
>  
>  	/* Semaphore interrupt disable and route to host */
>  	guc_route_semaphores(guc, false);
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.h b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.h
> index c57b29cdb1a6..a77de0d6ed58 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.h
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.h
> @@ -16,7 +16,7 @@ struct intel_engine_cs;
>  void intel_guc_submission_init_early(struct intel_guc *guc);
>  int intel_guc_submission_init(struct intel_guc *guc);
>  int intel_guc_submission_enable(struct intel_guc *guc);
> -void intel_guc_submission_disable(struct intel_guc *guc);
> +void intel_guc_submission_disable(struct intel_guc *guc, bool sync);
>  void intel_guc_submission_fini(struct intel_guc *guc);
>  int intel_guc_preempt_work_create(struct intel_guc *guc);
>  void intel_guc_preempt_work_destroy(struct intel_guc *guc);
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_uc.c b/drivers/gpu/drm/i915/gt/uc/intel_uc.c
> index 18250fb64bd8..5b76f0d4d2a6 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_uc.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_uc.c
> @@ -566,7 +566,7 @@ static int __uc_init_hw(struct intel_uc *uc)
>  	 * We've failed to load the firmware :(
>  	 */
>  err_submission:
> -	intel_guc_submission_disable(guc);
> +	intel_guc_submission_disable(guc, true);
>  err_log_capture:
>  	__uc_capture_load_err_log(uc);
>  err_rps:
> @@ -597,7 +597,7 @@ static void __uc_fini_hw(struct intel_uc *uc)
>  		return;
>  
>  	if (intel_uc_uses_guc_submission(uc))
> -		intel_guc_submission_disable(guc);
> +		intel_guc_submission_disable(guc, true);
>  
>  	__uc_sanitize(uc);
>  }
> -- 
> 2.34.1



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