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 14:08:54 +0200
From:   Thomas Hellstrom <thellstrom@...are.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.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

Resending hopefully better formatted..


On 06/14/2018 01:49 PM, Andrea Parri wrote:
> [...]
>
>>>> +		/*
>>>> +		 * 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

No you are probably not misunderstanding me at all. My comment 
originated from the reading of the kerneldoc of set_current_state()

/*
* set_current_state() includes a barrier so that the write of current->state
* is correctly serialised wrt the caller's subsequent test of whether to
* actually sleep:
*
* for (;;) {
* set_current_state(TASK_UNINTERRUPTIBLE);
* if (!need_sleep)
* break;
*
* schedule();
* }
* __set_current_state(TASK_RUNNING);
*
* If the caller does not need such serialisation (because, for instance, the
* condition test and condition change and wakeup are under the same 
lock) then
* use __set_current_state().
*
* The above is typically ordered against the wakeup, which does:
*
* need_sleep = false;
* wake_up_state(p, TASK_UNINTERRUPTIBLE);
*
* Where wake_up_state() (and all other wakeup primitives) imply enough
* barriers to order the store of the variable against wakeup.
---
*/

And with ctx->wounded := !need_sleep this exactly matches what's 
happening in my code. So what I was trying to say in the comment was 
that this above contract is sufficient to guarantee the "condition" 
above, whitout me actually knowing exactly what barriers are required. 
Thanks, Thomas