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

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Date:   Thu, 19 Mar 2020 01:51:09 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Logan Gunthorpe <logang@...tatee.com>,
        Kurt Schwemmer <kurt.schwemmer@...rosemi.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [patch V2 08/15] Documentation: Add lock ordering and nesting
 documentation

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