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

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Message-Id: <0B1950D8-9319-4F25-B14B-4ED949A57BE0@joelfernandes.org>
Date:   Tue, 20 Dec 2022 21:41:17 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.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 7:50 PM, Frederic Weisbecker <frederic@...nel.org> 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.

Sure I see what you are saying and it’s a valid point as well. However why do you need memory barrier D (labeled such in the kernel code) for that? You already have a memory barrier A before the lock count is read. That will suffice for the ordering pairing with the addr dependency.
In other words, if updater sees readers lock counts, then reader would be making those lock count updates on post-flip inactive index, not the one being scanned as you wanted, and you will accomplish that just with the mem barrier A.

So D fixes the above issue you are talking about (lock count update), however that is already fixed by the memory barrier A. But you still need D for the issue I mentioned (unlock counts vs flip).

That’s just my opinion and let’s discuss more because I cannot rule out that I am missing something with this complicated topic ;-)

Thanks.