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SubjectRe: Locking question for DRM
> > 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


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