linux-kernel - Re: [RFC PATCH] fs: elide the smp_rmb fence in fd

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Message-ID: <CAHk-=wjNb1G19p3efTsD9SmM3PzWdde1K2=nYb6OUgUdmmgS=g@mail.gmail.com>
Date: Thu, 5 Dec 2024 11:26:35 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: paulmck@...nel.org
Cc: Mateusz Guzik <mjguzik@...il.com>, Al Viro <viro@...iv.linux.org.uk>, brauner@...nel.org, 
	jack@...e.cz, linux-fsdevel@...r.kernel.org, edumazet@...gle.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: elide the smp_rmb fence in fd_install()

On Thu, 5 Dec 2024 at 10:41, Paul E. McKenney <paulmck@...nel.org> wrote:
>
> > To my understanding this is the idiomatic way of spelling out the
> > non-existent in Linux smp_consume_load, for the resize_in_progress
> > flag.
>
> In Linus, "smp_consume_load()" is named rcu_dereference().

Linux.

But yes and no.

It's worth making it really really clear that "rcu_dereference()" is
*not* just a different name for some "smp_consume_load()" operation.

Why? Because a true smp_consume_load() would work with any random kind
of flags etc. And rcu_dereference() works only because it's a pointer,
and there's an inherent data dependency to what the result points to.

Paul obviously knows this, but let's make it very clear in this
discussion, because if somebody decided "I want a smp_consume_load(),
and I'll use rcu_dereference() to do that", the end result would
simply not work for arbitrary data, like a flags field or something,
where comparing it against a value will only result in a control
dependency, not an actual data dependency.

             Linus