linux-kernel - Re: [PATCH -next] locking/osq_lock: annotate a data race in osq

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Date:   Tue, 11 Feb 2020 06:57:05 -0500
From:   Qian Cai <cai@....pw>
To:     Marco Elver <elver@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] locking/osq_lock: annotate a data race in osq_lock



> On Feb 11, 2020, at 5:16 AM, Marco Elver <elver@...gle.com> wrote:
> 
> I have said this before: we're not just guarding against load/store
> tearing, although on their own, they make it deceptively easy to
> reason about data races.
> 
> The case here seems to be another instance of a C-CAS, to avoid
> unnecessarily dirtying a cacheline.
> 
> Here, the loop would make me suspicious, because a compiler could
> optimize out re-loading the value. Due to the smp_load_acquire,
> however, at the least we have 1 implied compiler barrier in this loop
> which means that will likely not happen.
> 
> Before jumping to 'data_race()', I would ask again: how bad is the
> READ_ONCE? Is the generated code the same? If so, just use the
> READ_ONCE. Do you want to reason about all compiler optimizations? For
> this code here, I certainly don't want to.
> 
> But in the end it's up to what maintainers prefer, and maybe there is
> a very compelling argument that I missed that makes the fact this is a
> data race always safe.

Yes, I feel like locking maintainers prefer data_race() rather than blindly adding READ_ONCE() unless there is an strong evidence that the later is needed.

Since I can’t prove it is strictly needed to prevent from which specific optimization, I had chosen the data_race() approach.