linux-kernel - Re: [PATCH v2] make buffer_locked provide an acquire semantics

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Message-ID: <YucGwGr4KTPPxbYJ@casper.infradead.org>
Date:   Sun, 31 Jul 2022 23:48:32 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Will Deacon <will@...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 v2] make buffer_locked provide an acquire semantics

On Sun, Jul 31, 2022 at 10:30:11AM -0700, Paul E. McKenney wrote:
> That said, I confess that I am having a hard time finding the
> buffer_locked() definition.  So if buffer_locked() uses normal C-language
> accesses to sample the BH_Lock bit of the ->b_state field, then yes,
> there could be a problem.  The compiler would then be free to reorder
> calls to buffer_locked() because it could then assume that no other
> thread was touching that ->b_state field.

You're having a hard time finding it because it's constructed with the C
preprocessor.  I really wish we generated header files using CPP once
and then included the generated/ file.  That would make them greppable.

You're looking for include/linux/buffer_head.h and it's done like this:

enum bh_state_bits {
...
        BH_Lock,        /* Is locked */
...

#define BUFFER_FNS(bit, name)                                           \
...
static __always_inline int buffer_##name(const struct buffer_head *bh)  \
{                                                                       \
        return test_bit(BH_##bit, &(bh)->b_state);                      \
}

BUFFER_FNS(Lock, locked)

(fwiw, the page/folio versions of these functions don't autogenerate
the lock or uptodate ones because they need extra functions called)