linux-kernel - Re: [RFC] arm64: Enforce observed order for spinlock and data

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Message-ID: <884bd5d3a9a1bcf2a276130ffc17412a@codeaurora.org>
Date:   Tue, 04 Oct 2016 13:53:35 -0400
From:   bdegraaf@...eaurora.org
To:     Mark Rutland <mark.rutland@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Timur Tabi <timur@...eaurora.org>,
        Nathan Lynch <nathan_lynch@...tor.com>,
        linux-kernel@...r.kernel.org,
        Christopher Covington <cov@...eaurora.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC] arm64: Enforce observed order for spinlock and data

On 2016-10-04 06:12, Mark Rutland wrote:
> On Mon, Oct 03, 2016 at 03:20:57PM -0400, bdegraaf@...eaurora.org 
> wrote:
>> On 2016-10-01 14:11, Mark Rutland wrote:
>> >Hi Brent,
>> >
>> >Evidently my questions weren't sufficiently clear; even with your
>> >answers it's not clear to me what precise issue you're attempting to
>> >solve.  I've tried to be more specific this time.
>> >
>> >At a high-level, can you clarify whether you're attempting to solve is:
>> >
>> >(a) a functional correctness issue (e.g. data corruption)
>> >(b) a performance issue
>> >
>> >And whether this was seen in practice, or found through code
>> >inspection?
> 
>> Thinking about this, as the reader/writer code has no known "abuse"
>> case, I'll remove it from the patchset, then provide a v2 patchset
>> with a detailed explanation for the lockref problem using the commits
>> you provided as an example, as well as performance consideration.
> 
> If there's a functional problem, let's consider that in isolation 
> first.
> Once we understand that, then we can consider doing what is optimal.
> 
> As should be obvious from the above, I'm confused because this patch
> conflates functional details with performance optimisations which (to
> me) sound architecturally dubious.
> 
> I completely agree with Peter that if the problem lies with lockref, it
> should be solved in the lockref code.
> 
> Thanks,
> Mark.

After looking at this, the problem is not with the lockref code per se: 
it is
a problem with arch_spin_value_unlocked().  In the out-of-order case,
arch_spin_value_unlocked() can return TRUE for a spinlock that is in 
fact
locked but the lock is not observable yet via an ordinary load.  Other 
than
ensuring order on the locking side (as the prior patch did), there is a 
way
to make arch_spin_value_unlock's TRUE return value deterministic, but it
requires that it does a write-back to the lock to ensure we didn't 
observe
the unlocked value while another agent was in process of writing back a
locked value.

Brent