linux-kernel - Re: [PATCH v5] add barriers to buffer functions

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Date:   Sun, 7 Aug 2022 15:50:14 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 v5] add barriers to buffer functions

On Sun, Aug 07, 2022 at 07:37:22AM -0400, Mikulas Patocka wrote:
> @@ -135,6 +133,49 @@ BUFFER_FNS(Meta, meta)
>  BUFFER_FNS(Prio, prio)
>  BUFFER_FNS(Defer_Completion, defer_completion)
>  
> +static __always_inline void set_buffer_uptodate(struct buffer_head *bh)
> +{
> +	/*
> +	 * make it consistent with folio_mark_uptodate
> +	 * pairs with smp_acquire__after_ctrl_dep in buffer_uptodate
> +	 */
> +	smp_wmb();
> +	set_bit(BH_Uptodate, &bh->b_state);
> +}
> +
> +static __always_inline void clear_buffer_uptodate(struct buffer_head *bh)
> +{
> +	clear_bit(BH_Uptodate, &bh->b_state);
> +}
> +
> +static __always_inline int buffer_uptodate(const struct buffer_head *bh)
> +{
> +	bool ret = test_bit(BH_Uptodate, &bh->b_state);
> +	/*
> +	 * make it consistent with folio_test_uptodate
> +	 * pairs with smp_wmb in set_buffer_uptodate
> +	 */
> +	if (ret)
> +		smp_acquire__after_ctrl_dep();
> +	return ret;
> +}

This all works for me.  While we have the experts paying attention,
would it be better to do

	return smp_load_acquire(&bh->b_state) & (1L << BH_Uptodate) > 0;

> +static __always_inline void set_buffer_locked(struct buffer_head *bh)
> +{
> +	set_bit(BH_Lock, &bh->b_state);
> +}
> +
> +static __always_inline int buffer_locked(const struct buffer_head *bh)
> +{
> +	bool ret = test_bit(BH_Lock, &bh->b_state);
> +	/*
> +	 * pairs with smp_mb__after_atomic in unlock_buffer
> +	 */
> +	if (!ret)
> +		smp_acquire__after_ctrl_dep();
> +	return ret;
> +}

Are there places that think that lock/unlock buffer implies a memory
barrier?