Re: Locking question for DRM

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

> > I don't think that works. drm_open_helper doesn't appear to be under the
> > BKL merely the struct mutex.
> 
> It blocks the case you specified of two releases happening together.

But not parallel open/release

> > But setmaster can sleep so the BKL is dropped on contention of the
> > struct_mutex, ditto dropmaster
> 
> they should only sleep in the mutex lock nuless the driver callback is
> allocating memory. but yeah its a bit of a mess.

With the mutex alone the damage is done. Consider two setmasters and some
other action which is making the mutex contend

		CPU1				CPU2

file->priv->minor->master == NULL ?
file_priv->minor->master != file_priv->master

mutex_lock	(drop BKL)
				file->priv->minor->master === NULL
				master != file_priv->master
				mutex_lock   [drop BKL]
takes mutex
minor->master = drm_master_get
is_master = 1
master_set
drop mutex
return 0
				takes mutex
				minor->master = drm_master_get
				is_master = 1
				master_set
				drops mutex
				return 0


Alan