On Thu, Jul 22, 2021 at 6:00 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> 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 ;-)
The use case is essentially 2 nesting locks, and only the innermost is
used to update a field. So when you only read this field, it's safe if
either of these two locks are held. Essentially this is a read/write lock
type of thing, except for various reasons the two locks might not be of
the same type (like here where the write lock is a mutex, but the read
lock is a spinlock).
It's a bit like the rcu_derefence macro where it's ok to either be in a
rcu_read_lock() section, or holding the relevant lock that's used to
update the value. We do _not_ have two different locks that allow writing
to the same X.
Does that make it clearer what's the use-case here?
In an example:
void * interesting_pointer.
do_update_interesting_pointer()
{
mutex_lock(A);
/* do more stuff to prepare things */
spin_lock(B);
interesting_pointer = new_value;
spin_unlock(B);
mutex_unlock(A);
}
read_interesting_thing_locked()
{
lockdep_assert_held_either(A, B);
return interesting_pointer->thing;
}
read_interesting_thing()
{
int thing;
spin_lock(B);
thing = interesting_pointer->thing;
spin_unlock(B);
return B;
}
spinlock might also be irqsafe here if this can be called from irq
context.
Cheers, Daniel
> 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
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
