drm: Push drm_global_mutex locking in drm_open

We want to only take the BKL on crap drivers, but to know whether
we have a crap driver we first need to look it up. Split this shuffle
out from the main BKL-disabling patch, for more clarity. Historical
aside: When the kernel-wide BKL was removed, it was replaced by
drm_global_mutex within the scope of the drm subsystem hence why these
two things are (almost) interchangeable as concepts here.

Since the minors are refcounted drm_minor_acquire is purely internal
and this does not have a driver visible effect.

v2: Push the locking even further into drm_open(), suggested by Chris.
This gives us more symmetry with drm_release(), and maybe a futuer
avenue where we make drm_global_mutex locking (partially) opt-in like
with drm_release_noglobal().

v3:
- Actually push this stuff correctly, don't unlock twice (Chris)
- Fix typo on commit message, plus explain why BKL = drm_global_mutex
  (Sam)

Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204150146.2006481-5-daniel.vetter@ffwll.ch

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 7c18a98..b8f716e 100644 (file)
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -1079,17 +1079,14 @@ static int drm_stub_open(struct inode *inode, struct file *filp)
 
        DRM_DEBUG("\n");
 
-       mutex_lock(&drm_global_mutex);
        minor = drm_minor_acquire(iminor(inode));
-       if (IS_ERR(minor)) {
-               err = PTR_ERR(minor);
-               goto out_unlock;
-       }
+       if (IS_ERR(minor))
+               return PTR_ERR(minor);
 
        new_fops = fops_get(minor->dev->driver->fops);
        if (!new_fops) {
                err = -ENODEV;
-               goto out_release;
+               goto out;
        }
 
        replace_fops(filp, new_fops);
@@ -1098,10 +1095,9 @@ static int drm_stub_open(struct inode *inode, struct file *filp)
        else
                err = 0;
 
-out_release:
+out:
        drm_minor_release(minor);
-out_unlock:
-       mutex_unlock(&drm_global_mutex);
+
        return err;
 }
 

diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index 1075b3a..80d5564 100644 (file)
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -378,6 +378,8 @@ int drm_open(struct inode *inode, struct file *filp)
        if (IS_ERR(minor))
                return PTR_ERR(minor);
 
+       mutex_lock(&drm_global_mutex);
+
        dev = minor->dev;
        if (!atomic_fetch_inc(&dev->open_count))
                need_setup = 1;
@@ -395,10 +397,14 @@ int drm_open(struct inode *inode, struct file *filp)
                        goto err_undo;
                }
        }
+
+       mutex_unlock(&drm_global_mutex);
+
        return 0;
 
 err_undo:
        atomic_dec(&dev->open_count);
+       mutex_unlock(&drm_global_mutex);
        drm_minor_release(minor);
        return retcode;
 }