Re: [PATCH] drm: Give the DRM device's anon_inode a unique lockclass for its mmap_rswem

On Mon, Dec 11, 2017 at 11:39 AM, Chris Wilson chris@chris-wilson.co.uk wrote:

Teach lockdep to track the device's internal mmapping separately from the generic lockclass over all other inodes. Since this is device private we wish to allow a different locking hierarchy than is typified by the requirement for the mmap_rwsem being the outermost lock for handling pagefaults. By giving the internal mmap_rwsem a distinct lockclass, lockdep can identify it and learn/enforce its distinct locking requirements.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104209 Signed-off-by: Chris Wilson chris@chris-wilson.co.uk Cc: Daniel Vetter daniel.vetter@ffwll.ch

I think both the commit message and comment are a bit too fluffy - the critical bit is that we're biting ourselves on gtt mmaps from usersptr, and that's explicitly not allowed exactly because it would deadlock.

I'm also not sure it's a good idea to implement this in generic code, since this is a very i915 specific issue, and other drivers (who might be a lot less sloppy here) will now no longer get reports about this deadlock.

Aside from that I'm not really sure why you think the bugzilla link is a false positive: The mapping->rwsem is the one for the gtt in both cases I think. -Daniel

---

drivers/gpu/drm/drm_drv.c | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c index 9acc1e157813..21ad06c3d684 100644 --- a/drivers/gpu/drm/drm_drv.c +++ b/drivers/gpu/drm/drm_drv.c @@ -393,6 +393,7 @@ static struct file_system_type drm_fs_type = {

static struct inode *drm_fs_inode_new(void) {

•   static struct lock_class_key lockclass;
    struct inode *inode;
    int r;
  
  

@@ -403,8 +404,22 @@ static struct inode *drm_fs_inode_new(void) }

    inode = alloc_anon_inode(drm_fs_mnt->mnt_sb);

•   if (IS_ERR(inode))
  

•   if (IS_ERR(inode)) {
            simple_release_fs(&drm_fs_mnt, &drm_fs_cnt);
  

•           return inode;
  

•   }
  

•   /*
  

•    * Teach lockdep to track the device's internal mmapping separately
  

•    * from all other inodes. Since this is device private we wish to
  

•    * allow a different locking hierarchy than is typified by the
  

•    * requirement for the mmap_rwsem being the outermost lock for
  

•    * handling pagefaults. By giving the internal mmap_rwsem a distinct
  

•    * lockclass, lockdep can identify it and thereby learn and enforce its
  

•    * distinct locking requirements.
  

•    */
  

•   lockdep_set_class_and_name(&inode->i_mapping->i_mmap_rwsem,
  

•                              &lockclass, "drm_fs_inode");
  
    return inode;
  

}

2.15.1