Re: [PATCH v2 02/12] device-core: Add dev->lock_class to enable device_lock() lockdep validation

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 14, 2022 at 10:17:13AM -0700, Dan Williams wrote:

> One more sanity check... So in driver subsystems there are cases where
> a device on busA hosts a topology on busB. When that happens there's a
> need to set the lock class late in a driver since busA knows nothing
> about the locking rules of busB.

I'll pretend I konw what you're talking about ;-)

> Since the device has a longer lifetime than a driver when the driver
> exits it must set dev->mutex back to the novalidate class, otherwise
> it sets up a use after free of the static lock_class_key.

I'm not following, static storage has infinite lifetime.

> I came up with this and it seems to work, just want to make sure I'm
> properly using the lock_set_class() API and it is ok to transition
> back and forth from the novalidate case:
> 
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index 990b6670222e..32673e1a736d 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -405,6 +405,29 @@ struct cxl_nvdimm_bridge
> *cxl_find_nvdimm_bridge(struct cxl_nvdimm *cxl_nvd);
>  #define __mock static
>  #endif
> 
> +#ifdef CONFIG_PROVE_LOCKING
> +static inline void cxl_lock_reset_class(void *_dev)
> +{
> +       struct device *dev = _dev;
> +
> +       lock_set_class(&dev->mutex.dep_map, "__lockdep_no_validate__",
> +                      &__lockdep_no_validate__, 0, _THIS_IP_);
> +}
> +
> +static inline int cxl_lock_set_class(struct device *dev, const char *name,
> +                                    struct lock_class_key *key)
> +{
> +       lock_set_class(&dev->mutex.dep_map, name, key, 0, _THIS_IP_);
> +       return devm_add_action_or_reset(dev, cxl_lock_reset_class, dev);
> +}
> +#else
> +static inline int cxl_lock_set_class(struct device *dev, const char *name,
> +                                    struct lock_class_key *key)
> +{
> +       return 0;
> +}
> +#endif

Under the assumption that the lock is held (lock_set_class() will
actually barf if @lock isn't held) this should indeed work as expected
(although I think you got the @name part 'wrong', I think that's
canonically something like "&dev->mutex" or something).