In a future patch, a read lock on drm_device.master_rwsem is
held in the ioctl handler before the check for ioctl
permissions. However, this produces the following lockdep splat:
======================================================
WARNING: possible circular locking dependency detected
5.14.0-rc6-CI-Patchwork_20831+ #1 Tainted: G U
------------------------------------------------------
kms_lease/1752 is trying to acquire lock:
ffffffff827bad88 (drm_global_mutex){+.+.}-{3:3}, at: drm_open+0x64/0x280
but task is already holding lock:
ffff88812e350108 (&dev->master_rwsem){++++}-{3:3}, at:
drm_ioctl_kernel+0xfb/0x1a0
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&dev->master_rwsem){++++}-{3:3}:
lock_acquire+0xd3/0x310
down_read+0x3b/0x140
drm_master_internal_acquire+0x1d/0x60
drm_client_modeset_commit+0x10/0x40
__drm_fb_helper_restore_fbdev_mode_unlocked+0x88/0xb0
drm_fb_helper_set_par+0x34/0x40
intel_fbdev_set_par+0x11/0x40 [i915]
fbcon_init+0x270/0x4f0
visual_init+0xc6/0x130
do_bind_con_driver+0x1de/0x2c0
do_take_over_console+0x10e/0x180
do_fbcon_takeover+0x53/0xb0
register_framebuffer+0x22d/0x310
__drm_fb_helper_initial_config_and_unlock+0x36c/0x540
intel_fbdev_initial_config+0xf/0x20 [i915]
async_run_entry_fn+0x28/0x130
process_one_work+0x26d/0x5c0
worker_thread+0x37/0x390
kthread+0x13b/0x170
ret_from_fork+0x1f/0x30
-> #1 (&helper->lock){+.+.}-{3:3}:
lock_acquire+0xd3/0x310
__mutex_lock+0xa8/0x930
__drm_fb_helper_restore_fbdev_mode_unlocked+0x44/0xb0
intel_fbdev_restore_mode+0x2b/0x50 [i915]
drm_lastclose+0x27/0x50
drm_release_noglobal+0x42/0x60
__fput+0x9e/0x250
task_work_run+0x6b/0xb0
exit_to_user_mode_prepare+0x1c5/0x1d0
syscall_exit_to_user_mode+0x19/0x50
do_syscall_64+0x46/0xb0
entry_SYSCALL_64_after_hwframe+0x44/0xae
-> #0 (drm_global_mutex){+.+.}-{3:3}:
validate_chain+0xb39/0x1e70
__lock_acquire+0x5a1/0xb70
lock_acquire+0xd3/0x310
__mutex_lock+0xa8/0x930
drm_open+0x64/0x280
drm_stub_open+0x9f/0x100
chrdev_open+0x9f/0x1d0
do_dentry_open+0x14a/0x3a0
dentry_open+0x53/0x70
drm_mode_create_lease_ioctl+0x3cb/0x970
drm_ioctl_kernel+0xc9/0x1a0
drm_ioctl+0x201/0x3d0
__x64_sys_ioctl+0x6a/0xa0
do_syscall_64+0x37/0xb0
entry_SYSCALL_64_after_hwframe+0x44/0xae
other info that might help us debug this:
Chain exists of:
drm_global_mutex --> &helper->lock --> &dev->master_rwsem
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&dev->master_rwsem);
lock(&helper->lock);
lock(&dev->master_rwsem);
lock(drm_global_mutex);
*** DEADLOCK ***
The lock hierarchy inversion happens because we grab the
drm_global_mutex while already holding on to master_rwsem. To avoid
this, we do some prep work to grab the drm_global_mutex before
checking for ioctl permissions.
At the same time, we update the check for the global mutex to use the
drm_dev_needs_global_mutex helper function.
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
---
drivers/gpu/drm/drm_ioctl.c | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 880fc565d599..2cb57378a787 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -779,19 +779,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
if (drm_dev_is_unplugged(dev))
return -ENODEV;
+ /* Enforce sane locking for modern driver ioctls. */
+ if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED))
+ mutex_lock(&drm_global_mutex);
+
retcode = drm_ioctl_permit(flags, file_priv);
if (unlikely(retcode))
- return retcode;
+ goto out;
- /* Enforce sane locking for modern driver ioctls. */
- if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) ||
- (flags & DRM_UNLOCKED))
- retcode = func(dev, kdata, file_priv);
- else {
- mutex_lock(&drm_global_mutex);
- retcode = func(dev, kdata, file_priv);
+ retcode = func(dev, kdata, file_priv);
+
+out:
+ if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED))
mutex_unlock(&drm_global_mutex);
- }
return retcode;
}
EXPORT_SYMBOL(drm_ioctl_kernel);
--
2.25.1
