linux-kernel - Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

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Message-Id: <6BD506EA-77D2-4AC9-87A5-C8781594CF0F@joelfernandes.org>
Date:   Tue, 20 Dec 2022 23:26:12 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC 0/2] srcu: Remove pre-flip memory barrier



> On Dec 20, 2022, at 10:43 PM, Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
> On 2022-12-20 19:58, Frederic Weisbecker wrote:
>>> On Wed, Dec 21, 2022 at 01:49:57AM +0100, Frederic Weisbecker wrote:
>>> On Tue, Dec 20, 2022 at 07:15:00PM -0500, Joel Fernandes wrote:
>>>> On Tue, Dec 20, 2022 at 5:45 PM Frederic Weisbecker <frederic@...nel.org> wrote:
>>>> Agreed about (1).
>>>> 
>>>>> _ In (2), E pairs with the address-dependency between idx and lock_count.
>>>> 
>>>> But that is not the only reason. If that was the only reason for (2),
>>>> then there is an smp_mb() just before the next-scan post-flip before
>>>> the lock counts are read.
>>> 
>>> The post-flip barrier makes sure the new idx is visible on the next READER's
>>> turn, but it doesn't protect against the fact that "READ idx then WRITE lock[idx]"
>>> may appear unordered from the update side POV if there is no barrier between the
>>> scan and the flip.
>>> 
>>> If you remove the smp_mb() from the litmus test I sent, things explode.
>> Or rather, look at it the other way, if there is no barrier between the lock
>> scan and the index flip (E), then the index flip can appear to be written before the
>> lock is read. Which means you may start activating the index before you finish
>> reading it (at least it appears that way from the readers pont of view).
> 
> Considering that you can have pre-existing readers from arbitrary index appearing anywhere in the grace period (because a reader can fetch the
> index and be preempted for an arbitrary amount of time before incrementing the lock count), the grace period algorithm needs to deal with the fact that a newcoming reader can appear in a given index either before or after the flip.
> 
> I don't see how flipping the index before or after loading the unlock/lock values would break anything (except for unlikely counter overflow situations as previously discussed).

If you say unlikely, that means it can happen some times which is bad enough ;-). Maybe you mean impossible. I would not settle for anything less than keeping the memory barrier around if it helps unlikely cases, but only D does help for the theoretical wrapping/overflow issue. E is broken and does not even help the theoretical issue IMO. And both D and E do not affect correctness IMO.

Anyway in all likelihood, I will be trying to remove E completely and clarify docs on D in the coming weeks. And also try to drop the size of the counters per our discussions

Thanks.



> 
> Thanks,
> 
> Mathieu
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>