Re: [PATCH 3/4] drm: Push drm_global_mutex locking in drm_open

Quoting Daniel Vetter (2020-01-28 10:46:00)

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.

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_globale_mutex locking (partially) opt-in like with drm_release_noglobal().

Cc: Chris Wilson chris@chris-wilson.co.uk Signed-off-by: Daniel Vetter daniel.vetter@intel.com

---

drivers/gpu/drm/drm_drv.c | 14 +++++--------- drivers/gpu/drm/drm_file.c | 6 ++++++ 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c index 8deff75b484c..05bdf0b9d2b3 100644 --- a/drivers/gpu/drm/drm_drv.c +++ b/drivers/gpu/drm/drm_drv.c @@ -1085,17 +1085,14 @@ 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);
  

@@ -1104,10 +1101,9 @@ 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 1075b3a8b5b1..d36cb74ebe0c 100644 --- 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_unlock(&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);
  

The only reason why I could think it was in drm_stub_open() not drm_open() was for the possibility of some driver using a different callback. Such a driver would not be partaking in the drm_global_mutex so... Reviewed-by: Chris Wilson chris@chris-wilson.co.uk -Chris