On Thu, Jul 22, 2021 at 12:38:10PM +0200, Daniel Vetter wrote:
> On Thu, Jul 22, 2021 at 05:29:27PM +0800, Desmond Cheong Zhi Xi wrote:
> > Inside drm_is_current_master, using the outer drm_device.master_mutex
> > to protect reads of drm_file.master makes the function prone to creating
> > lock hierarchy inversions. Instead, we can use the
> > drm_file.master_lookup_lock that sits at the bottom of the lock
> > hierarchy.
> >
> > Reported-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/drm_auth.c | 9 +++++----
> > 1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> > index f00354bec3fb..9c24b8cc8e36 100644
> > --- a/drivers/gpu/drm/drm_auth.c
> > +++ b/drivers/gpu/drm/drm_auth.c
> > @@ -63,8 +63,9 @@
> >
> > static bool drm_is_current_master_locked(struct drm_file *fpriv)
> > {
> > - lockdep_assert_held_once(&fpriv->minor->dev->master_mutex);
> > -
> > + /* Either drm_device.master_mutex or drm_file.master_lookup_lock
> > + * should be held here.
> > + */
>
> Disappointing that lockdep can't check or conditions for us, a
> lockdep_assert_held_either would be really neat in some cases.
>
The implementation is not hard but I don't understand the usage, for
example, if we have a global variable x, and two locks L1 and L2, and
the function
void do_something_to_x(void)
{
lockdep_assert_held_either(L1, L2);
x++;
}
and two call sites:
void f(void)
{
lock(L1);
do_something_to_x();
unlock(L1);
}
void g(void)
{
lock(L2);
do_something_to_x();
unlock(L2);
}
, wouldn't it be racy if f() and g() called by two threads at the same
time? Usually I would expect there exists a third synchronazition
mechanism (say M), which synchronizes the calls to f() and g(), and we
put M in the lockdep_assert_held() check inside do_something_to_x()
like:
void do_something_to_x(void)
{
lockdep_assert_held_once(M);
x++;
}
But of course, M may not be a lock, so we cannot put the assert there.
My cscope failed to find ->master_lookup_lock in -rc2 and seems it's not
introduced in the patchset either, could you point me the branch this
patchset is based on, so that I could understand this better, and maybe
come up with a solution? Thanks ;-)
Regards,
Boqun
> Adding lockdep folks, maybe they have ideas.
>
> On the patch:
>
> Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
>
> > return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> > }
> >
> > @@ -82,9 +83,9 @@ bool drm_is_current_master(struct drm_file *fpriv)
> > {
> > bool ret;
> >
> > - mutex_lock(&fpriv->minor->dev->master_mutex);
> > + spin_lock(&fpriv->master_lookup_lock);
> > ret = drm_is_current_master_locked(fpriv);
> > - mutex_unlock(&fpriv->minor->dev->master_mutex);
> > + spin_unlock(&fpriv->master_lookup_lock);
> >
> > return ret;
> > }
> > --
> > 2.25.1
> >
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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