Re: [PATCH] Revert "drm: add a locked version of drm_is_current_master"

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On 22/6/21 3:54 pm, Daniel Vetter wrote:
This reverts commit 1815d9c86e3090477fbde066ff314a7e9721ee0f.

Unfortunately this inverts the locking hierarchy, so back to the
drawing board. Full lockdep splat below:

======================================================
WARNING: possible circular locking dependency detected
5.13.0-rc7-CI-CI_DRM_10254+ #1 Not tainted
------------------------------------------------------
kms_frontbuffer/1087 is trying to acquire lock:
ffff88810dcd01a8 (&dev->master_mutex){+.+.}-{3:3}, at: drm_is_current_master+0x1b/0x40
but task is already holding lock:
ffff88810dcd0488 (&dev->mode_config.mutex){+.+.}-{3:3}, at: drm_mode_getconnector+0x1c6/0x4a0
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&dev->mode_config.mutex){+.+.}-{3:3}:
        __mutex_lock+0xab/0x970
        drm_client_modeset_probe+0x22e/0xca0
        __drm_fb_helper_initial_config_and_unlock+0x42/0x540
        intel_fbdev_initial_config+0xf/0x20 [i915]
        async_run_entry_fn+0x28/0x130
        process_one_work+0x26d/0x5c0
        worker_thread+0x37/0x380
        kthread+0x144/0x170
        ret_from_fork+0x1f/0x30
-> #1 (&client->modeset_mutex){+.+.}-{3:3}:
        __mutex_lock+0xab/0x970
        drm_client_modeset_commit_locked+0x1c/0x180
        drm_client_modeset_commit+0x1c/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+0x1e5/0x2d0
        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+0x37/0x380
        kthread+0x144/0x170
        ret_from_fork+0x1f/0x30
-> #0 (&dev->master_mutex){+.+.}-{3:3}:
        __lock_acquire+0x151e/0x2590
        lock_acquire+0xd1/0x3d0
        __mutex_lock+0xab/0x970
        drm_is_current_master+0x1b/0x40
        drm_mode_getconnector+0x37e/0x4a0
        drm_ioctl_kernel+0xa8/0xf0
        drm_ioctl+0x1e8/0x390
        __x64_sys_ioctl+0x6a/0xa0
        do_syscall_64+0x39/0xb0
        entry_SYSCALL_64_after_hwframe+0x44/0xae
other info that might help us debug this:
Chain exists of: &dev->master_mutex --> &client->modeset_mutex --> &dev->mode_config.mutex
  Possible unsafe locking scenario:
        CPU0                    CPU1
        ----                    ----
   lock(&dev->mode_config.mutex);
                                lock(&client->modeset_mutex);
                                lock(&dev->mode_config.mutex);
   lock(&dev->master_mutex);
*** DEADLOCK ***

Hi Daniel,

Just a thought.

Since &dev->mode_config.mutex is the modeset BKL and its scope isn't clear, keeping the dependency as is would mean that any lock that depends on &dev->mode_config.mutex would also depend on &client->modeset_mutex. Seems like this might be vulnerable to more circular dependencies.

Would it make sense to invert the locking dependency for &client->modeset_mutex and &dev->mode_config.mutex to become &dev->mode_config.mutex --> &client->modeset_mutex? Something like this:

diff --git a/drivers/gpu/drm/drm_client_modeset.c b/drivers/gpu/drm/drm_client_modeset.c
index ced09c7c06f9..859f99d97cde 100644
--- a/drivers/gpu/drm/drm_client_modeset.c
+++ b/drivers/gpu/drm/drm_client_modeset.c
@@ -811,9 +811,9 @@ int drm_client_modeset_probe(struct drm_client_dev *client, unsigned int width,
 		goto out;
 	}

+	mutex_lock(&dev->mode_config.mutex);
 	mutex_lock(&client->modeset_mutex);

-	mutex_lock(&dev->mode_config.mutex);
 	for (i = 0; i < connector_count; i++)
total_modes_count += connectors[i]->funcs->fill_modes(connectors[i], width, height);
 	if (!total_modes_count)
@@ -838,7 +838,6 @@ int drm_client_modeset_probe(struct drm_client_dev *client, unsigned int width,
 		drm_client_pick_crtcs(client, connectors, connector_count,
 				      crtcs, modes, 0, width, height);
 	}
-	mutex_unlock(&dev->mode_config.mutex);

 	drm_client_modeset_release(client);

@@ -869,6 +868,7 @@ int drm_client_modeset_probe(struct drm_client_dev *client, unsigned int width,
 	}

 	mutex_unlock(&client->modeset_mutex);
+	mutex_unlock(&dev->mode_config.mutex);
 out:
 	kfree(crtcs);
 	kfree(modes);
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index f6baa2046124..74302d110609 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1443,8 +1443,8 @@ static int pan_display_legacy(struct fb_var_screeninfo *var,
 	struct drm_mode_set *modeset;
 	int ret = 0;

-	mutex_lock(&client->modeset_mutex);
 	drm_modeset_lock_all(fb_helper->dev);
+	mutex_lock(&client->modeset_mutex);
 	drm_client_for_each_modeset(modeset, client) {
 		modeset->x = var->xoffset;
 		modeset->y = var->yoffset;
@@ -1457,8 +1457,8 @@ static int pan_display_legacy(struct fb_var_screeninfo *var,
 			}
 		}
 	}
-	drm_modeset_unlock_all(fb_helper->dev);
 	mutex_unlock(&client->modeset_mutex);
+	drm_modeset_unlock_all(fb_helper->dev);

 	return ret;
 }

1 lock held by kms_frontbuffer/1087:
  #0: ffff88810dcd0488 (&dev->mode_config.mutex){+.+.}-{3:3}, at: drm_mode_getconnector+0x1c6/0x4a0
stack backtrace:
CPU: 7 PID: 1087 Comm: kms_frontbuffer Not tainted 5.13.0-rc7-CI-CI_DRM_10254+ #1
Hardware name: Intel Corporation Ice Lake Client Platform/IceLake U DDR4 SODIMM PD RVP TLC, BIOS ICLSFWR1.R00.3234.A01.1906141750 06/14/2019
Call Trace:
  dump_stack+0x7f/0xad
  check_noncircular+0x12e/0x150
  __lock_acquire+0x151e/0x2590
  lock_acquire+0xd1/0x3d0
  __mutex_lock+0xab/0x970
  drm_is_current_master+0x1b/0x40
  drm_mode_getconnector+0x37e/0x4a0
  drm_ioctl_kernel+0xa8/0xf0
  drm_ioctl+0x1e8/0x390
  __x64_sys_ioctl+0x6a/0xa0
  do_syscall_64+0x39/0xb0
  entry_SYSCALL_64_after_hwframe+0x44/0xae

daniel@phenom:~/linux/drm-misc-fixes$ dim fixes 1815d9c86e3090477fbde066ff314a7e9721ee0f
Fixes: 1815d9c86e30 ("drm: add a locked version of drm_is_current_master")
Cc: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
Cc: Emil Velikov <emil.l.velikov@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
---
  drivers/gpu/drm/drm_auth.c | 51 ++++++++++++++------------------------
  1 file changed, 19 insertions(+), 32 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 86d4b72e95cb..232abbba3686 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -61,35 +61,6 @@
   * trusted clients.
   */
-static bool drm_is_current_master_locked(struct drm_file *fpriv)
-{
-	lockdep_assert_held_once(&fpriv->master->dev->master_mutex);
-
-	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
-}
-
-/**
- * drm_is_current_master - checks whether @priv is the current master
- * @fpriv: DRM file private
- *
- * Checks whether @fpriv is current master on its device. This decides whether a
- * client is allowed to run DRM_MASTER IOCTLs.
- *
- * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
- * - the current master is assumed to own the non-shareable display hardware.
- */
-bool drm_is_current_master(struct drm_file *fpriv)
-{
-	bool ret;
-
-	mutex_lock(&fpriv->master->dev->master_mutex);
-	ret = drm_is_current_master_locked(fpriv);
-	mutex_unlock(&fpriv->master->dev->master_mutex);
-
-	return ret;
-}
-EXPORT_SYMBOL(drm_is_current_master);
-
  int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
  {
  	struct drm_auth *auth = data;
@@ -252,7 +223,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
  	if (ret)
  		goto out_unlock;
- if (drm_is_current_master_locked(file_priv))
+	if (drm_is_current_master(file_priv))
  		goto out_unlock;
if (dev->master) {
@@ -301,7 +272,7 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
  	if (ret)
  		goto out_unlock;
- if (!drm_is_current_master_locked(file_priv)) {
+	if (!drm_is_current_master(file_priv)) {
  		ret = -EINVAL;
  		goto out_unlock;
  	}
@@ -350,7 +321,7 @@ void drm_master_release(struct drm_file *file_priv)
  	if (file_priv->magic)
  		idr_remove(&file_priv->master->magic_map, file_priv->magic);
- if (!drm_is_current_master_locked(file_priv))
+	if (!drm_is_current_master(file_priv))
  		goto out;
drm_legacy_lock_master_cleanup(dev, master);
@@ -371,6 +342,22 @@ void drm_master_release(struct drm_file *file_priv)
  	mutex_unlock(&dev->master_mutex);
  }
+/**
+ * drm_is_current_master - checks whether @priv is the current master
+ * @fpriv: DRM file private
+ *
+ * Checks whether @fpriv is current master on its device. This decides whether a
+ * client is allowed to run DRM_MASTER IOCTLs.
+ *
+ * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
+ * - the current master is assumed to own the non-shareable display hardware.
+ */
+bool drm_is_current_master(struct drm_file *fpriv)
+{
+	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
+}
+EXPORT_SYMBOL(drm_is_current_master);
+
  /**
   * drm_master_get - reference a master pointer
   * @master: &struct drm_master


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