Re: [PATCH 2/2] drm/i915: Use rcu instead of stop_machine in set_wedged

[Chris Wilson]

Quoting Daniel Vetter (2017-10-06 10:06:37)
> 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 20e4933c478a1ca694b38fa4ac44d99e659941f5
> Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> 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.
> 
> 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.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102886
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103096
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Marta Lofstedt <marta.lofstedt@xxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> ---
>  drivers/gpu/drm/i915/i915_gem.c                   | 31 +++++++++--------------
>  drivers/gpu/drm/i915/i915_gem_request.c           |  2 ++
>  drivers/gpu/drm/i915/selftests/i915_gem_request.c |  2 ++
>  3 files changed, 16 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index ab8c6946fea4..e79a6ca60265 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3020,16 +3020,8 @@ static void nop_submit_request(struct drm_i915_gem_request *request)
>         intel_engine_init_global_seqno(request->engine, request->global_seqno);
>  }
>  
> -static void engine_set_wedged(struct intel_engine_cs *engine)
> +static void engine_complete_requests(struct intel_engine_cs *engine)
>  {
> -       /* We need to be sure that no thread is running the old callback as
> -        * we install the nop handler (otherwise we would submit a request
> -        * to hardware that will never complete). In order to prevent this
> -        * race, we wait until the machine is idle before making the swap
> -        * (using stop_machine()).
> -        */
> -       engine->submit_request = nop_submit_request;
> -
>         /* Mark all executing requests as skipped */
>         engine->cancel_requests(engine);

How are we planning to serialise the intel_engine_init_global_seqno()
here with the in-flight nop_submit? With sufficient thrust we will get a
stale breadcrumb and an incomplete request.
-Chris