stop_machine is not really a locking primitive we should use, except
when the hw folks tell us the hw is broken and that's the only way to
work around it.

This patch tries to address the locking abuse of stop_machine() from

commit 20e4933c
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Nov 22 14:41:21 2016 +0000

    drm/i915: Stop the machine as we install the wedged submit_request handler

Chris said parts of the reasons for going with stop_machine() was that
it's no overhead for the fast-path. But these callbacks use irqsave
spinlocks and do a bunch of MMIO, and rcu_read_lock is _real_ fast.

To stay as close as possible to the stop_machine semantics we first
update all the submit function pointers to the nop handler, then call
synchronize_rcu() to make sure no new requests can be submitted. This
should give us exactly the huge barrier we want.

I pondered whether we should annotate engine->submit_request as __rcu
and use rcu_assign_pointer and rcu_dereference on it. But the reason
behind those is to make sure the compiler/cpu barriers are there for
when you have an actual data structure you point at, to make sure all
the writes are seen correctly on the read side. But we just have a
function pointer, and .text isn't changed, so no need for these
barriers and hence no need for annotations.

Unfortunately there's a complication with the call to
intel_engine_init_global_seqno:

- Without stop_machine we must hold the corresponding spinlock.

- Without stop_machine we must ensure that all requests are marked as
  having failed with dma_fence_set_error() before we call it. That
  means we need to split the nop request submission into two phases,
  both synchronized with rcu:

  1. Only stop submitting the requests to hw and mark them as failed.

  2. After all pending requests in the scheduler/ring are suitably
  marked up as failed and we can force complete them all, also force
  complete by calling intel_engine_init_global_seqno().

This should fix the followwing lockdep splat:

======================================================
WARNING: possible circular locking dependency detected
4.14.0-rc3-CI-CI_DRM_3179+ #1 Tainted: G     U
------------------------------------------------------
kworker/3:4/562 is trying to acquire lock:
 (cpu_hotplug_lock.rw_sem){++++}, at: [<ffffffff8113d4bc>] stop_machine+0x1c/0x40

but task is already holding lock:
 (&dev->struct_mutex){+.+.}, at: [<ffffffffa0136588>] i915_reset_device+0x1e8/0x260 [i915]

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #6 (&dev->struct_mutex){+.+.}:
       __lock_acquire+0x1420/0x15e0
       lock_acquire+0xb0/0x200
       __mutex_lock+0x86/0x9b0
       mutex_lock_interruptible_nested+0x1b/0x20
       i915_mutex_lock_interruptible+0x51/0x130 [i915]
       i915_gem_fault+0x209/0x650 [i915]
       __do_fault+0x1e/0x80
       __handle_mm_fault+0xa08/0xed0
       handle_mm_fault+0x156/0x300
       __do_page_fault+0x2c5/0x570
       do_page_fault+0x28/0x250
       page_fault+0x22/0x30

-> #5 (&mm->mmap_sem){++++}:
       __lock_acquire+0x1420/0x15e0
       lock_acquire+0xb0/0x200
       __might_fault+0x68/0x90
       _copy_to_user+0x23/0x70
       filldir+0xa5/0x120
       dcache_readdir+0xf9/0x170
       iterate_dir+0x69/0x1a0
       SyS_getdents+0xa5/0x140
       entry_SYSCALL_64_fastpath+0x1c/0xb1

-> #4 (&sb->s_type->i_mutex_key#5){++++}:
       down_write+0x3b/0x70
       handle_create+0xcb/0x1e0
       devtmpfsd+0x139/0x180
       kthread+0x152/0x190
       ret_from_fork+0x27/0x40

-> #3 ((complete)&req.done){+.+.}:
       __lock_acquire+0x1420/0x15e0
       lock_acquire+0xb0/0x200
       wait_for_common+0x58/0x210
       wait_for_completion+0x1d/0x20
       devtmpfs_create_node+0x13d/0x160
       device_add+0x5eb/0x620
       device_create_groups_vargs+0xe0/0xf0
       device_create+0x3a/0x40
       msr_device_create+0x2b/0x40
       cpuhp_invoke_callback+0xc9/0xbf0
       cpuhp_thread_fun+0x17b/0x240
       smpboot_thread_fn+0x18a/0x280
       kthread+0x152/0x190
       ret_from_fork+0x27/0x40

-> #2 (cpuhp_state-up){+.+.}:
       __lock_acquire+0x1420/0x15e0
       lock_acquire+0xb0/0x200
       cpuhp_issue_call+0x133/0x1c0
       __cpuhp_setup_state_cpuslocked+0x139/0x2a0
       __cpuhp_setup_state+0x46/0x60
       page_writeback_init+0x43/0x67
       pagecache_init+0x3d/0x42
       start_kernel+0x3a8/0x3fc
       x86_64_start_reservations+0x2a/0x2c
       x86_64_start_kernel+0x6d/0x70
       verify_cpu+0x0/0xfb

-> #1 (cpuhp_state_mutex){+.+.}:
       __lock_acquire+0x1420/0x15e0
       lock_acquire+0xb0/0x200
       __mutex_lock+0x86/0x9b0
       mutex_lock_nested+0x1b/0x20
       __cpuhp_setup_state_cpuslocked+0x53/0x2a0
       __cpuhp_setup_state+0x46/0x60
       page_alloc_init+0x28/0x30
       start_kernel+0x145/0x3fc
       x86_64_start_reservations+0x2a/0x2c
       x86_64_start_kernel+0x6d/0x70
       verify_cpu+0x0/0xfb

-> #0 (cpu_hotplug_lock.rw_sem){++++}:
       check_prev_add+0x430/0x840
       __lock_acquire+0x1420/0x15e0
       lock_acquire+0xb0/0x200
       cpus_read_lock+0x3d/0xb0
       stop_machine+0x1c/0x40
       i915_gem_set_wedged+0x1a/0x20 [i915]
       i915_reset+0xb9/0x230 [i915]
       i915_reset_device+0x1f6/0x260 [i915]
       i915_handle_error+0x2d8/0x430 [i915]
       hangcheck_declare_hang+0xd3/0xf0 [i915]
       i915_hangcheck_elapsed+0x262/0x2d0 [i915]
       process_one_work+0x233/0x660
       worker_thread+0x4e/0x3b0
       kthread+0x152/0x190
       ret_from_fork+0x27/0x40

other info that might help us debug this:

Chain exists of:
  cpu_hotplug_lock.rw_sem --> &mm->mmap_sem --> &dev->struct_mutex

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&dev->struct_mutex);
                               lock(&mm->mmap_sem);
                               lock(&dev->struct_mutex);
  lock(cpu_hotplug_lock.rw_sem);

 *** DEADLOCK ***

3 locks held by kworker/3:4/562:
 #0:  ("events_long"){+.+.}, at: [<ffffffff8109c64a>] process_one_work+0x1aa/0x660
 #1:  ((&(&i915->gpu_error.hangcheck_work)->work)){+.+.}, at: [<ffffffff8109c64a>] process_one_work+0x1aa/0x660
 #2:  (&dev->struct_mutex){+.+.}, at: [<ffffffffa0136588>] i915_reset_device+0x1e8/0x260 [i915]

stack backtrace:
CPU: 3 PID: 562 Comm: kworker/3:4 Tainted: G     U          4.14.0-rc3-CI-CI_DRM_3179+ #1
Hardware name:                  /NUC7i5BNB, BIOS BNKBL357.86A.0048.2017.0704.1415 07/04/2017
Workqueue: events_long i915_hangcheck_elapsed [i915]
Call Trace:
 dump_stack+0x68/0x9f
 print_circular_bug+0x235/0x3c0
 ? lockdep_init_map_crosslock+0x20/0x20
 check_prev_add+0x430/0x840
 ? irq_work_queue+0x86/0xe0
 ? wake_up_klogd+0x53/0x70
 __lock_acquire+0x1420/0x15e0
 ? __lock_acquire+0x1420/0x15e0
 ? lockdep_init_map_crosslock+0x20/0x20
 lock_acquire+0xb0/0x200
 ? stop_machine+0x1c/0x40
 ? i915_gem_object_truncate+0x50/0x50 [i915]
 cpus_read_lock+0x3d/0xb0
 ? stop_machine+0x1c/0x40
 stop_machine+0x1c/0x40
 i915_gem_set_wedged+0x1a/0x20 [i915]
 i915_reset+0xb9/0x230 [i915]
 i915_reset_device+0x1f6/0x260 [i915]
 ? gen8_gt_irq_ack+0x170/0x170 [i915]
 ? work_on_cpu_safe+0x60/0x60
 i915_handle_error+0x2d8/0x430 [i915]
 ? vsnprintf+0xd1/0x4b0
 ? scnprintf+0x3a/0x70
 hangcheck_declare_hang+0xd3/0xf0 [i915]
 ? intel_runtime_pm_put+0x56/0xa0 [i915]
 i915_hangcheck_elapsed+0x262/0x2d0 [i915]
 process_one_work+0x233/0x660
 worker_thread+0x4e/0x3b0
 kthread+0x152/0x190
 ? process_one_work+0x660/0x660
 ? kthread_create_on_node+0x40/0x40
 ret_from_fork+0x27/0x40
Setting dangerous option reset - tainting kernel
i915 0000:00:02.0: Resetting chip after gpu hang
Setting dangerous option reset - tainting kernel
i915 0000:00:02.0: Resetting chip after gpu hang

v2: Have 1 global synchronize_rcu() barrier across all engines, and
improve commit message.

v3: We need to protect the seqno update with the timeline spinlock (in
set_wedged) to avoid racing with other updates of the seqno, like we
already do in nop_submit_request (Chris).

v4: Use two-phase sequence to plug the race Chris spotted where we can
complete requests before they're marked up with -EIO.

v5: Review from Chris:
- simplify nop_submit_request.
- Add comment to rcu_read_lock section.
- Align comments with the new style.

v6: Remove unused variable to appease CI.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102886
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103096
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marta Lofstedt <marta.lofstedt@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171011091019.1425-1-daniel.vetter@ffwll.ch