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

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Message-ID: <425f4d5b-d414-de46-f388-55f1e6821ba6@vmware.com>
Date:   Thu, 14 Jun 2018 15:43:04 +0200
From:   Thomas Hellstrom <thellstrom@...are.com>
To:     Matthew Wilcox <willy@...radead.org>
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 06/14/2018 03:29 PM, Matthew Wilcox wrote:
> 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?

It's intended to be enforced by storing the algorithm choice in the 
WW_MUTEX_CLASS which must be common for an acquire context and the 
ww_mutexes it acquires. However, I don't think there is a check that 
that holds. I guess we could add it as a DEBUG_MUTEX test in 
ww_mutex_lock().

Thanks,

Thomas