Re: [RFC tip/locking/lockdep v5 04/17] lockdep: Introduce lock_list::dep

Peter Zijlstra
2018-02-24T08:38:14Z

On Sat, Feb 24, 2018 at 02:30:05PM +0800, Boqun Feng wrote:
> On Sat, Feb 24, 2018 at 01:32:50PM +0800, Boqun Feng wrote:

> > /*
> > * DEP_*_BIT in lock_list::dep
> > *
> > * For dependency @prev -> @next:
> > *
> > * RR: both @prev and @next are recursive read locks, i.e. ->read == 2.
> > * RN: @prev is recursive and @next is non-recursive.
> > * NR: @prev is a not recursive and @next is recursive.
> > * NN: both @prev and @next are non-recursive.
> > *
> > * Note that we define the value of DEP_*_BITs so that:
> > * bit0 is prev->read != 2
> > * bit1 is next->read != 2
> > */
> > #define DEP_RR_BIT 0
> > #define DEP_RN_BIT 1
> > #define DEP_NR_BIT 2
> > #define DEP_NN_BIT 3
> >
> > #define DEP_RR_MASK (1U << (DEP_RR_BIT))
> > #define DEP_RN_MASK (1U << (DEP_RN_BIT))
> > #define DEP_NR_MASK (1U << (DEP_NR_BIT))
> > #define DEP_NN_MASK (1U << (DEP_NN_BIT))
> >
> > static inline unsigned int
> > __calc_dep_bit(struct held_lock *prev, struct held_lock *next)
> > {
> > return (prev->read != 2) + ((next->read != 2) << 1)
> > }
> >
> > static inline u8 calc_dep(struct held_lock *prev, struct held_lock *next)
> > {
> > return 1U << __calc_dep_bit(prev, next);
> > }
> >
> > static inline bool only_rx(u8 dep)
> > {
> > return !(dep & (DEP_NR_MASK | DEP_NN_MASK));
> > }
> >
> > static inline bool only_xr(u8 dep)
> > {
> > return !(dep & (DEP_NR_MASK | DEP_NN_MASK));
> > }
> >

> > > > if (have_xr && is_rx(entry->dep))
> > > > continue;
> > > >
> > > > entry->have_xr = is_xr(entry->dep);
> > > >
>
> Hmm.. I think this part also needs some tweak:
>
> /* if -> prev is *R, and we only have R* for prev -> this, * skip*/
> if (have_xr && only_rx(entry->dep))
> continue;
>
> /*
> * we pick a *R for prev -> this only if:
> * prev -> this dependencies are all *R
> * or
> * -> prev is *R, and we don't have NN for prev -> this
> */
> entry->have_xr = only_xr(entry->dep) || (have_xr && !is_nn(entry->dep));
>
> otherwise, we will wrongly set entry->have_xr to false if have_xr is
> true and we have RN for prev -> this.

OK, so its saturday morning and such, but what? Why should we set
have_xr true when we have RN? Note that if we only had RN we'd already
have bailed on the continue due to only_rx().

So can you elaborate a bit?