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

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@gmail.com Cc: Emil Velikov emil.l.velikov@gmail.com Cc: stable@vger.kernel.org Signed-off-by: Daniel Vetter daniel.vetter@intel.com Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com Cc: Maxime Ripard mripard@kernel.org Cc: Thomas Zimmermann tzimmermann@suse.de Cc: David Airlie airlied@linux.ie Cc: Daniel Vetter daniel@ffwll.ch

---

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