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

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Oct 06, 2017 at 12:03:49PM +0100, Chris Wilson wrote:
> 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@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.
> > 
> > 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@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>
> > ---
> >  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.

Yeah that part looks indeed fishy. Well the entire "let the nop handler
fake-complete requests" logic is something I don't really understand. I
guess there's an exclusive relationship between requests handled directly
(and cancelled in engine->cancel_request) and requests with external
dma_fence dependencies.

But then I'm not really seeing what I'm changing, since even with the stop
machine you might end up with a bunch of requests depending upon external
fences, which then all complete at roughly the same time and race multiple
calls to intel_engine_init_global_seqno with each another.

With the fake submission, do we really need to call intel_engine_init_global_seqno?

So yeah, no idea, but pretty sure I didn't make it worse.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch