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Subject[PATCH] drm/i915: Preallocate mmu notifier to unbreak cpu hotplug deadlock
Date
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-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

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>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

--

Patch itself pretty much untested, I just want to figure out whether
we should fix this (and similar backtraces going through
msr_create_device) in i915, or whether the cpu hotplug folks will take
care of them all.

Afaict this is not because of changes in 4.14-rc1, but really only due
to the new cross-release checks.

Note that the above trace is on top of -rc3 (plus plenty of drm/i915
stuff), so should have Peter's recent lockdep fixes for cpu up vs.
down already.
-Daniel
---
drivers/gpu/drm/i915/i915_gem_userptr.c | 65 +++++++++++++++------------------
1 file changed, 29 insertions(+), 36 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 2d4996de7331..afe166921572 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -160,36 +160,6 @@ static const struct mmu_notifier_ops i915_gem_userptr_notifier = {
.invalidate_range_start = i915_gem_userptr_mn_invalidate_range_start,
};

-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)
- return ERR_PTR(-ENOMEM);
-
- spin_lock_init(&mn->lock);
- mn->mn.ops = &i915_gem_userptr_notifier;
- mn->objects = RB_ROOT_CACHED;
- mn->wq = alloc_workqueue("i915-userptr-release", WQ_UNBOUND, 0);
- if (mn->wq == NULL) {
- kfree(mn);
- 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;
-}
-
static void
i915_gem_userptr_release__mmu_notifier(struct drm_i915_gem_object *obj)
{
@@ -210,23 +180,46 @@ 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 = mm->mn, *free_mn;
+ int ret;

mn = mm->mn;
if (mn)
return mn;

+ free_mn = kmalloc(sizeof(*free_mn), GFP_KERNEL);
+ if (free_mn == NULL)
+ return ERR_PTR(-ENOMEM);
+
+ spin_lock_init(&free_mn->lock);
+ free_mn->mn.ops = &i915_gem_userptr_notifier;
+ free_mn->objects = RB_ROOT_CACHED;
+ /* must allocate wq outside of mm->mmap_sem */
+ free_mn->wq = alloc_workqueue("i915-userptr-release", WQ_UNBOUND, 0);
+ if (free_mn->wq == NULL) {
+ ret = -ENOMEM;
+ goto err_kfree;
+ }
+
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) */
+ ret = __mmu_notifier_register(&free_mn->mn, mm->mm);
+ if (ret == 0)
+ mm->mn = mn =free_mn;
+ } else {
+ mn = mm->mn;
}
mutex_unlock(&mm->i915->mm_lock);
up_write(&mm->mm->mmap_sem);

- return mn;
+err_wq:
+ destroy_workqueue(free_mn->wq);
+err_kfree:
+ kfree(free_mn);
+
+ return ret == 0 ? mn : ERR_PTR(ret);
}

static int
--
2.14.1
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