linux-kernel - Re: [GIT PULL] RCU changes for v6.7

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Date:   Mon, 30 Oct 2023 18:12:51 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Neeraj Upadhyay <neeraj.upadhyay@....com>,
        Uladzislau Rezki <urezki@...il.com>,
        Z qiang <qiang.zhang1211@...il.com>
Subject: Re: [GIT PULL] RCU changes for v6.7

On Fri, 27 Oct 2023 at 01:33, Frederic Weisbecker <frederic@...nel.org> wrote:
>
> rcu/stall: Stall detection updates. Introduce RCU CPU Stall notifiers
>         that allows a subsystem to provide informations to help debugging.
>         Also cure some false positive stalls.

I absolutely detest this stall notifier thing.

Putting the stall notifier before the stall message does not "help
debugging". Quite the reverse. It ends up being a lovely way to make
sure that the debug message is never printed, because there's some
entirely untested - and thus buggy - notifier on the chain before the
printout from the actual stall code.

I've pulled this, but I really want to voice my objection against
these kinds of "debugging aids". I have personally spent way too many
hours debugging a dead machine because some "debug aid" ended up being
untested garbage.

If you absolutely think that this is a worthy and useful thing to do,
then at the very least make sure that these "debug aids" will always
come *after* the core output, and can't make things horrendously
worse.

But in general, think twice before adding "maybe somebody else wants
to print debug info". Because unless you have a really really REALLY
good reason for it, it's more likely to hurt than to help.

Right now I see no users of this except for the rcu torture code, and
it certainly doesn't seem hugely important there. And so I'm wondering
what the actual real use-case would be.

                  Linus