linux-kernel - Re: [PATCH v3 2/2] make buffer_locked provide an acquire semantics

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Date:   Sun, 31 Jul 2022 13:51:21 -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 2/2] make buffer_locked provide an acquire semantics

On Sun, Jul 31, 2022 at 1:43 PM Mikulas Patocka <mpatocka@...hat.com> wrote:
>
> +
> +static __always_inline int buffer_locked(const struct buffer_head *bh)
> +{
> +       unsigned long state = smp_load_acquire(&bh->b_state);
> +       return test_bit(BH_Lock, &state);

This should not use 'test_bit()'. I suspect that generates horrendous
code, because it's a volatile access, so now you'll load it into a
register, and I suspect it will generate s pointless spill just to do
a volatile load.

I didn't check.

So once you've loaded b_state, just test the bit directly with

    return (state & (1u << BH_Lock)) != 0;

or whatever.

              Linus