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

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Date:   Mon, 1 Aug 2022 06:40:20 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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, 31 Jul 2022, Linus Torvalds wrote:

> 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?

None of the CPUs have speculative writes - so the write can't be moved 
before the "test_bit" function. So, we are only concerned about reads.

> 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

OK - I'm sending new patches that introduce the function test_bit_acquire.

Mikulas