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Subject[PATCH 1/2] drm/i915: Preallocate our mmu notifier workequeu 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-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;
+
+ 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
--
2.14.1
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