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

[Daniel Vetter]

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.

Maybe we should update the comment of drm_global_mutex to explain what it
protects and why.

I'm also confused how this patch connects to the splat, since for i915 we
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

> 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
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch