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

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Message-ID: <f4199d1f-306c-681f-8bb9-26d66ecf5121@efficios.com>
Date:   Tue, 20 Dec 2022 22:43:25 -0500
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     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 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).

Thanks,

Mathieu

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