Re: [PATCH 1/2] drm/i915: Preallocate our mmu notifier workequeu to unbreak cpu hotplug deadlock

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Daniel Vetter (2017-10-06 10:06:36)
> 4.14-rc1 gained the fancy new cross-release support in lockdep, which
> seems to have uncovered a few more rules about what is allowed and
> isn't.
> 
> This one here seems to indicate that allocating a work-queue while
> holding mmap_sem is a no-go, so let's try to preallocate it.
> 
> Of course another way to break this chain would be somewhere in the
> cpu hotplug code, since this isn't the only trace we're finding now
> which goes through msr_create_device.
> 
> Full lockdep splat:
> 
> ======================================================
> WARNING: possible circular locking dependency detected
> 4.14.0-rc1-CI-CI_DRM_3118+ #1 Tainted: G     U
> ------------------------------------------------------
> prime_mmap/1551 is trying to acquire lock:
>  (cpu_hotplug_lock.rw_sem){++++}, at: [<ffffffff8109dbb7>] apply_workqueue_attrs+0x17/0x50
> 
> but task is already holding lock:
>  (&dev_priv->mm_lock){+.+.}, at: [<ffffffffa01a7b2a>] i915_gem_userptr_init__mmu_notifier+0x14a/0x270 [i915]
> 
> which lock already depends on the new lock.
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #6 (&dev_priv->mm_lock){+.+.}:
>        __lock_acquire+0x1420/0x15e0
>        lock_acquire+0xb0/0x200
>        __mutex_lock+0x86/0x9b0
>        mutex_lock_nested+0x1b/0x20
>        i915_gem_userptr_init__mmu_notifier+0x14a/0x270 [i915]
>        i915_gem_userptr_ioctl+0x222/0x2c0 [i915]
>        drm_ioctl_kernel+0x69/0xb0
>        drm_ioctl+0x2f9/0x3d0
>        do_vfs_ioctl+0x94/0x670
>        SyS_ioctl+0x41/0x70
>        entry_SYSCALL_64_fastpath+0x1c/0xb1
> 
> -> #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+0xa3/0x840
>        cpuhp_thread_fun+0x7a/0x150
>        smpboot_thread_fn+0x18a/0x280
>        kthread+0x152/0x190
>        ret_from_fork+0x27/0x40
> 
> -> #2 (cpuhp_state){+.+.}:
>        __lock_acquire+0x1420/0x15e0
>        lock_acquire+0xb0/0x200
>        cpuhp_issue_call+0x10b/0x170
>        __cpuhp_setup_state_cpuslocked+0x134/0x2a0
>        __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+0x52/0x2a0
>        __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
>        apply_workqueue_attrs+0x17/0x50
>        __alloc_workqueue_key+0x1d8/0x4d9
>        i915_gem_userptr_init__mmu_notifier+0x1fb/0x270 [i915]
>        i915_gem_userptr_ioctl+0x222/0x2c0 [i915]
>        drm_ioctl_kernel+0x69/0xb0
>        drm_ioctl+0x2f9/0x3d0
>        do_vfs_ioctl+0x94/0x670
>        SyS_ioctl+0x41/0x70
>        entry_SYSCALL_64_fastpath+0x1c/0xb1
> 
> other info that might help us debug this:
> 
> Chain exists of:
>   cpu_hotplug_lock.rw_sem --> &mm->mmap_sem --> &dev_priv->mm_lock
> 
>  Possible unsafe locking scenario:
> 
>        CPU0                    CPU1
>        ----                    ----
>   lock(&dev_priv->mm_lock);
>                                lock(&mm->mmap_sem);
>                                lock(&dev_priv->mm_lock);
>   lock(cpu_hotplug_lock.rw_sem);
> 
>  *** DEADLOCK ***
> 
> 2 locks held by prime_mmap/1551:
>  #0:  (&mm->mmap_sem){++++}, at: [<ffffffffa01a7b18>] i915_gem_userptr_init__mmu_notifier+0x138/0x270 [i915]
>  #1:  (&dev_priv->mm_lock){+.+.}, at: [<ffffffffa01a7b2a>] i915_gem_userptr_init__mmu_notifier+0x14a/0x270 [i915]
> 
> stack backtrace:
> CPU: 4 PID: 1551 Comm: prime_mmap Tainted: G     U          4.14.0-rc1-CI-CI_DRM_3118+ #1
> Hardware name: Dell Inc. XPS 8300  /0Y2MRG, BIOS A06 10/17/2011
> Call Trace:
>  dump_stack+0x68/0x9f
>  print_circular_bug+0x235/0x3c0
>  ? lockdep_init_map_crosslock+0x20/0x20
>  check_prev_add+0x430/0x840
>  __lock_acquire+0x1420/0x15e0
>  ? __lock_acquire+0x1420/0x15e0
>  ? lockdep_init_map_crosslock+0x20/0x20
>  lock_acquire+0xb0/0x200
>  ? apply_workqueue_attrs+0x17/0x50
>  cpus_read_lock+0x3d/0xb0
>  ? apply_workqueue_attrs+0x17/0x50
>  apply_workqueue_attrs+0x17/0x50
>  __alloc_workqueue_key+0x1d8/0x4d9
>  ? __lockdep_init_map+0x57/0x1c0
>  i915_gem_userptr_init__mmu_notifier+0x1fb/0x270 [i915]
>  i915_gem_userptr_ioctl+0x222/0x2c0 [i915]
>  ? i915_gem_userptr_release+0x140/0x140 [i915]
>  drm_ioctl_kernel+0x69/0xb0
>  drm_ioctl+0x2f9/0x3d0
>  ? i915_gem_userptr_release+0x140/0x140 [i915]
>  ? __do_page_fault+0x2a4/0x570
>  do_vfs_ioctl+0x94/0x670
>  ? entry_SYSCALL_64_fastpath+0x5/0xb1
>  ? __this_cpu_preempt_check+0x13/0x20
>  ? trace_hardirqs_on_caller+0xe3/0x1b0
>  SyS_ioctl+0x41/0x70
>  entry_SYSCALL_64_fastpath+0x1c/0xb1
> RIP: 0033:0x7fbb83c39587
> RSP: 002b:00007fff188dc228 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: ffffffff81492963 RCX: 00007fbb83c39587
> RDX: 00007fff188dc260 RSI: 00000000c0186473 RDI: 0000000000000003
> RBP: ffffc90001487f88 R08: 0000000000000000 R09: 00007fff188dc2ac
> R10: 00007fbb83efcb58 R11: 0000000000000246 R12: 0000000000000000
> R13: 0000000000000003 R14: 00000000c0186473 R15: 00007fff188dc2ac
>  ? __this_cpu_preempt_check+0x13/0x20
> 
> v2: Set ret correctly when we raced with another thread.
> 
> v3: Use Chris' diff. Attach the right lockdep splat.
> 
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Sasha Levin <alexander.levin@verizon.com>
> Cc: Marta Lofstedt <marta.lofstedt@intel.com>
> Cc: Tejun Heo <tj@kernel.org>
> References: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_3180/shard-hsw3/igt@prime_mmap@test_userptr.html
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102939

The difference is that I would have s/Bugzilla/References/.
-Chris