Re: [PATCH] kthread: finer-grained lockdep/cross-release completion

[Peter Zijlstra]

On Thu, Dec 07, 2017 at 03:58:28PM +0100, Daniel Vetter wrote:

> [   85.069417] gem_exec_captur/2810 is trying to acquire lock:
> [   85.069419]  ((completion)&self->parked){+.+.}, at: [<ffffffff8109d69d>] kthread_park+0x3d/0x50
> [   85.069426] 
>                but task is already holding lock:
> [   85.069428]  (&dev->struct_mutex){+.+.}, at: [<ffffffffa039b13d>] i915_reset_device+0x1bd/0x230 [i915]
> [   85.069448] 
>                which lock already depends on the new lock.
> 
> [   85.069451] 
>                the existing dependency chain (in reverse order) is:
> [   85.069454] 
>                -> #3 (&dev->struct_mutex){+.+.}:
> [   85.069460]        __mutex_lock+0x81/0x9b0
> [   85.069481]        i915_mutex_lock_interruptible+0x47/0x130 [i915]
> [   85.069502]        i915_gem_fault+0x201/0x760 [i915]
> [   85.069507]        __do_fault+0x15/0x70
> [   85.069509]        __handle_mm_fault+0x7bf/0xda0
> [   85.069512]        handle_mm_fault+0x14f/0x2f0
> [   85.069515]        __do_page_fault+0x2d1/0x560
> [   85.069518]        page_fault+0x22/0x30
> [   85.069520] 
>                -> #2 (&mm->mmap_sem){++++}:
> [   85.069525]        __might_fault+0x63/0x90
> [   85.069529]        _copy_to_user+0x1e/0x70
> [   85.069532]        perf_read+0x21d/0x290
> [   85.069534]        __vfs_read+0x1e/0x120
> [   85.069536]        vfs_read+0xa1/0x150
> [   85.069539]        SyS_read+0x40/0xa0
> [   85.069541]        entry_SYSCALL_64_fastpath+0x1c/0x89

>                -> #0 ((completion)&self->parked){+.+.}:

> [   85.069692] Chain exists of:
>                  (completion)&self->parked --> &mm->mmap_sem --> &dev->struct_mutex

> [   85.069718] 3 locks held by gem_exec_captur/2810:

> [   85.069732]  #2:  (&dev->struct_mutex){+.+.}, at: [<ffffffffa039b13d>] i915_reset_device+0x1bd/0x230 [i915]

>                stack backtrace:

> [   85.069788]  lock_acquire+0xaf/0x200
> [   85.069793]  wait_for_common+0x54/0x210
> [   85.069807]  kthread_park+0x3d/0x50
> [   85.069827]  i915_gem_reset_prepare_engine+0x1d/0x90 [i915]
> [   85.069849]  i915_gem_reset_prepare+0x2c/0x60 [i915]
> [   85.069865]  i915_reset+0x66/0x230 [i915]
> [   85.069881]  i915_reset_device+0x1cb/0x230 [i915]
> [   85.069919]  i915_handle_error+0x2d3/0x430 [i915]
> [   85.069951]  i915_wedged_set+0x79/0xc0 [i915]
> [   85.069955]  simple_attr_write+0xab/0xc0
> [   85.069959]  full_proxy_write+0x4b/0x70
> [   85.069961]  __vfs_write+0x1e/0x130
> [   85.069976]  vfs_write+0xc0/0x1b0
> [   85.069979]  SyS_write+0x40/0xa0
> [   85.069982]  entry_SYSCALL_64_fastpath+0x1c/0x89


	Thread-A				k-Thread

	i915_reset_device
#3	  mutex_lock(&dev->struct_mutex)
	  i915_reset()
	    i915_gem_reset_prepare()
	      i915_gem_reset_prepare_engine()
	        kthread_park()

						__do_page_fault()
#2						  down_read(&mm->mmap_sem);
						  handle_mm_fault()
						    __handle_mm_fault()
						      __do_fault()
						        i915_gem_fault()
							  i915_mutex_lock_interruptible()
#3							    mutex_lock(&dev->struct_mutex)

							/* twiddles thumbs forever more */

#0		  wait_for_common()

#0						complete()


Is what it says I suppose. Now I don't know enough about that i915 code
to say if that breadcrumbs_signal thread can ever trigger a fault or
not. I got properly lost in that dma_fence callback maze.

You're saying not?


(also, that comment near need_resched() doesn't make sense to me)