Re: [patch V2 08/15] Documentation: Add lock ordering and nesting documentation

[Davidlohr Bueso]

On Wed, 18 Mar 2020, Thomas Gleixner wrote:
> +Owner semantics
> +===============
> +
> +Most lock types in the Linux kernel have strict owner semantics, i.e. the
> +context (task) which acquires a lock has to release it.
> +
> +There are two exceptions:
> +
> +  - semaphores
> +  - rwsems
> +
> +semaphores have no strict owner semantics for historical reasons. They are

I would rephrase this to:

semaphores have no owner semantics for historical reason, and as such
trylock and release operations can be called from interrupt context. They
are ...

> +often used for both serialization and waiting purposes. That's generally
> +discouraged and should be replaced by separate serialization and wait
> +mechanisms.
           ^ , such as mutexes or completions.

> +
> +rwsems have grown interfaces which allow non owner release for special
> +purposes. This usage is problematic on PREEMPT_RT because PREEMPT_RT
> +substitutes all locking primitives except semaphores with RT-mutex based
> +implementations to provide priority inheritance for all lock types except
> +the truly spinning ones. Priority inheritance on ownerless locks is
> +obviously impossible.
> +
> +For now the rwsem non-owner release excludes code which utilizes it from
> +being used on PREEMPT_RT enabled kernels. In same cases this can be
> +mitigated by disabling portions of the code, in other cases the complete
> +functionality has to be disabled until a workable solution has been found.

Thanks,
Davidlohr