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 06:29:05 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Thomas Hellstrom <thellstrom@...are.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.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

On Thu, Jun 14, 2018 at 01:54:15PM +0200, Thomas Hellstrom wrote:
> On 06/14/2018 01:36 PM, Peter Zijlstra wrote:
> > Currently you don't allow mixing WD and WW contexts (which is not
> > immediately obvious from the above code), and the above hard relies on
> > that. Are there sensible use cases for mixing them? IOW will your
> > current restriction stand without hassle?
> 
> Contexts _must_ agree on the algorithm used to resolve deadlocks. With
> Wait-Die, for example, older transactions will wait if a lock is held by a
> younger transaction and with Wound-Wait, younger transactions will wait if a
> lock is held by an older transaction so there is no way of mixing them.

Maybe the compiler should be enforcing that; ie make it a different type?