Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752582AbdLGT5Z (ORCPT <rfc822;w@1wt.eu>);
        Thu, 7 Dec 2017 14:57:25 -0500
Received: from bombadil.infradead.org ([65.50.211.133]:37910 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750922AbdLGT5X (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 7 Dec 2017 14:57:23 -0500
Date: Thu, 7 Dec 2017 20:57:09 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: LKML <linux-kernel@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        Tvrtko Ursulin <tvrtko.ursulin@intel.com>,
        Marta Lofstedt <marta.lofstedt@intel.com>,
        Byungchul Park <byungchul.park@lge.com>,
        Ingo Molnar <mingo@kernel.org>, Tejun Heo <tj@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>, Shaohua Li <shli@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jonathan Corbet <corbet@lwn.net>, Oleg Nesterov <oleg@redhat.com>,
        Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH] kthread: finer-grained lockdep/cross-release completion
Message-ID: <20171207195709.cxcil57boc6czdot@hirez.programming.kicks-ass.net>
References: <20171207100849.407-1-daniel.vetter@ffwll.ch>
 <20171207122255.zi5ishny24k66ik7@hirez.programming.kicks-ass.net>
 <20171207145828.z52kjbrrlcl75m7m@phenom.ffwll.local>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20171207145828.z52kjbrrlcl75m7m@phenom.ffwll.local>
User-Agent: NeoMutt/20170609 (1.8.3)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 3403
Lines: 93

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)