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

From: Davidlohr Bueso <[email-masked]>
Date: Thu, 19 Mar 2020 01:51:09 -0700

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