linux-kernel - Re: [PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes

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Date:   Thu, 14 Jun 2018 13:49:44 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Thomas Hellstrom <thellstrom@...are.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-doc@...r.kernel.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v2 1/2] locking: Implement an algorithm choice for
 Wound-Wait mutexes

[...]

> >>+		/*
> >>+		 * wake_up_process() paired with set_current_state() inserts
> >>+		 * sufficient barriers to make sure @owner either sees it's
> >>+		 * wounded or has a wakeup pending to re-read the wounded
> >>+		 * state.
> >IIUC, "sufficient barriers" = full memory barriers (here).  (You may
> >want to be more specific.)
> 
> Thanks for reviewing!
> OK. What about if someone relaxes that in the future?

This is actually one of my main concerns ;-)  as, IIUC, those barriers are
not only sufficient but also necessary: anything "less than a full barrier"
(in either wake_up_process() or set_current_state()) would _not_ guarantee
the "condition" above unless I'm misunderstanding it.

But am I misunderstanding it?  Which barriers/guarantee do you _need_ from
the above mentioned pairing? (hence my comment...)

  Andrea