[PATCH v3 7/9] drm: update global mutex lock in the ioctl handler

[Desmond Cheong Zhi Xi]

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 at gmail.com>
---
 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