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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 06/10/2017 10:06, Daniel Vetter wrote:
> 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
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_gem_userptr.c | 35 +++++++++++++++++++--------------
>   1 file changed, 20 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> index 2d4996de7331..f9b3406401af 100644
> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> @@ -164,7 +164,6 @@ static struct i915_mmu_notifier *
>   i915_mmu_notifier_create(struct mm_struct *mm)
>   {
>   	struct i915_mmu_notifier *mn;
> -	int ret;
>   
>   	mn = kmalloc(sizeof(*mn), GFP_KERNEL);
>   	if (mn == NULL)
> @@ -179,14 +178,6 @@ i915_mmu_notifier_create(struct mm_struct *mm)
>   		return ERR_PTR(-ENOMEM);
>   	}
>   
> -	 /* Protected by mmap_sem (write-lock) */
> -	ret = __mmu_notifier_register(&mn->mn, mm);
> -	if (ret) {
> -		destroy_workqueue(mn->wq);
> -		kfree(mn);
> -		return ERR_PTR(ret);
> -	}
> -
>   	return mn;
>   }
>   
> @@ -210,23 +201,37 @@ i915_gem_userptr_release__mmu_notifier(struct drm_i915_gem_object *obj)
>   static struct i915_mmu_notifier *
>   i915_mmu_notifier_find(struct i915_mm_struct *mm)
>   {
> -	struct i915_mmu_notifier *mn = mm->mn;
> +	struct i915_mmu_notifier *mn;
> +	int err;
>   
>   	mn = mm->mn;
>   	if (mn)
>   		return mn;
>   
> +	mn = i915_mmu_notifier_create(mm->mm);
> +	if (IS_ERR(mn))
> +		return mn;

Strictly speaking we don't want to fail just yet, only it we actually 
needed a new notifier and we failed to create it.

> +
> +	err = 0;
>   	down_write(&mm->mm->mmap_sem);
>   	mutex_lock(&mm->i915->mm_lock);
> -	if ((mn = mm->mn) == NULL) {
> -		mn = i915_mmu_notifier_create(mm->mm);
> -		if (!IS_ERR(mn))
> -			mm->mn = mn;
> +	if (mm->mn == NULL) {
> +		/* Protected by mmap_sem (write-lock) */
> +		err = __mmu_notifier_register(&mn->mn, mm->mm);
> +		if (!err) {
> +			/* Protected by mm_lock */
> +			mm->mn = fetch_and_zero(&mn);
> +		}
>   	}
>   	mutex_unlock(&mm->i915->mm_lock);
>   	up_write(&mm->mm->mmap_sem);
>   
> -	return mn;
> +	if (mn) {
> +		destroy_workqueue(mn->wq);
> +		kfree(mn);
> +	}
> +
> +	return err ? ERR_PTR(err) : mm->mn;
>   }
>   
>   static int
> 

Otherwise looks good to me.

I would also put a note in the commit on how working around the locking 
issue is also beneficial to performance with moving the allocation step 
outside the mmap_sem.

Regards,

Tvrtko
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