linux-kernel - Re: [PATCH v3 1/2] wait_bit: do read barrier after testing a bit

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Date:   Sun, 31 Jul 2022 13:57:16 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] wait_bit: do read barrier after testing a bit

On Sun, Jul 31, 2022 at 1:41 PM Mikulas Patocka <mpatocka@...hat.com> wrote:
>
> -       if (!test_bit(bit, word))
> +       if (!test_bit(bit, word)) {
> +               smp_rmb();

Logically, I don't think that makes sense.

Maybe you're checking the buffer being up-to-date before you *write* to it?

So smp_rmb() seems entirely wrong.

I think it should consistently aim for just doing

        unsigned long state = smp_read_acquire(word);
        if (!(state & (1 << bit)))
                return 0;

or whatever.

We should strive to *not* add new uses of the legacy memory barriers.
They are garbage from last century when people didn't know better.

Then people learnt to use acquire and release, and things improved.
Let's live in that improved world.

                 Linus