[PATCH v3 7/9] drm: update global mutex lock in the ioctl handler
Desmond Cheong Zhi Xi
desmondcheongzx at gmail.com
Thu Aug 19 10:52:58 UTC 2021
On 18/8/21 7:02 pm, Daniel Vetter wrote:
> On Wed, Aug 18, 2021 at 03:38:22PM +0800, Desmond Cheong Zhi Xi wrote:
>> 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.
>
> This is intentional, essentially we force all non-legacy drivers to have
> unlocked ioctl (otherwise everyone forgets to set that flag).
>
> For non-legacy drivers the global lock only ensures ordering between
> drm_open and lastclose (I think at least), and between
> drm_dev_register/unregister and the backwards ->load/unload callbacks
> (which are called in the wrong place, but we cannot fix that for legacy
> drivers).
>
> ->load/unload should be completely unused (maybe radeon still uses it),
> and ->lastclose is also on the decline.
>
Ah ok got it, I'll change the check back to
drm_core_check_feature(dev, DRIVER_LEGACY) then.
> Maybe we should update the comment of drm_global_mutex to explain what it
> protects and why.
>
The comments in drm_dev_needs_global_mutex make sense I think, I just
didn't read the code closely enough.
> I'm also confused how this patch connects to the splat, since for i915 we
Right, my bad, this is a separate instance of circular locking. I was
too hasty when I saw that for legacy drivers we might grab master_rwsem
then drm_global_mutex in the ioctl handler.
> shouldn't be taking the drm_global_lock here at all. The problem seems to
> be the drm_open_helper when we create a new lease, which is an entirely
> different can of worms.
>
> I'm honestly not sure how to best do that, but we should be able to create
> a file and then call drm_open_helper directly, or well a version of that
> which never takes the drm_global_mutex. Because that is not needed for
> nested drm_file opening:
> - legacy drivers never go down this path because leases are only supported
> with modesetting, and modesetting is only supported for non-legacy
> drivers
> - the races against dev->open_count due to last_close or ->load callbacks
> don't matter, because for the entire ioctl we already have an open
> drm_file and that wont disappear.
>
> So this should work, but I'm not entirely sure how to make it work.
> -Daniel
>
One idea that comes to mind is to change the outcome of
drm_dev_needs_global_mutex while we're in the ioctl, but that requires
more locking which sounds like a bad idea.
Another idea, which is quite messy, but just for thoughts, uses the idea
of pushing the master_rwsem read lock down:
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 7f523e1c5650..5d05e744b728 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -712,7 +712,7 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
DRM_RENDER_ALLOW),
DRM_IOCTL_DEF(DRM_IOCTL_CRTC_GET_SEQUENCE, drm_crtc_get_sequence_ioctl, 0),
DRM_IOCTL_DEF(DRM_IOCTL_CRTC_QUEUE_SEQUENCE, drm_crtc_queue_sequence_ioctl, 0),
- DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, DRM_MASTER),
+ DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, 0),
DRM_IOCTL_DEF(DRM_IOCTL_MODE_LIST_LESSEES, drm_mode_list_lessees_ioctl, DRM_MASTER),
DRM_IOCTL_DEF(DRM_IOCTL_MODE_GET_LEASE, drm_mode_get_lease_ioctl, DRM_MASTER),
DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl, DRM_MASTER),
diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
index 983701198ffd..a25bc69522b4 100644
--- a/drivers/gpu/drm/drm_lease.c
+++ b/drivers/gpu/drm/drm_lease.c
@@ -500,6 +500,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
return -EINVAL;
}
+ /* Clone the lessor file to create a new file for us */
+ DRM_DEBUG_LEASE("Allocating lease file\n");
+ lessee_file = file_clone_open(lessor_file);
+ if (IS_ERR(lessee_file))
+ return PTR_ERR(lessee_file);
+
+ down_read(&dev->master_rwsem);
+
+ if (!drm_is_current_master(lessor_priv)) {
+ ret = -EACCES;
+ goto out_file;
+ }
+
lessor = drm_file_get_master(lessor_priv);
/* Do not allow sub-leases */
if (lessor->lessor) {
@@ -547,14 +560,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
goto out_leases;
}
- /* Clone the lessor file to create a new file for us */
- DRM_DEBUG_LEASE("Allocating lease file\n");
- lessee_file = file_clone_open(lessor_file);
- if (IS_ERR(lessee_file)) {
- ret = PTR_ERR(lessee_file);
- goto out_lessee;
- }
-
lessee_priv = lessee_file->private_data;
/* Change the file to a master one */
drm_master_put(&lessee_priv->master);
@@ -571,17 +576,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
fd_install(fd, lessee_file);
drm_master_put(&lessor);
+ up_read(&dev->master_rwsem);
DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
return 0;
-out_lessee:
- drm_master_put(&lessee);
-
out_leases:
put_unused_fd(fd);
out_lessor:
drm_master_put(&lessor);
+
+out_file:
+ up_read(&dev->master_rwsem);
+ fput(lessee_file);
DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret);
return ret;
}
Something like this would also address the other deadlock we'd hit in
drm_mode_create_lease_ioctl():
drm_ioctl_kernel():
down_read(&master_rwsem); <--- down_read()
drm_mode_create_lease_ioctl():
drm_lease_create():
file_clone_open():
...
drm_open():
drm_open_helper():
drm_master_open():
down_write(&master_rwsem); <--- down_write()
Overall, I think the suggestion to push master_rwsem write locks down
into ioctls would solve the nesting problem for those ioctls.
Although I'm still a little concerned that, just like here, there might
be deeply embedded nested locking, so locking becomes prone to breaking.
It does smell a bit to me.
>> 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
>>
>
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