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

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Date:   Mon, 8 Aug 2022 10:26:10 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
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, 7 Aug 2022, Matthew Wilcox wrote:

> 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;

Yes, it may be nicer.

> > +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?

There's this in fs/reiserfs:

if (!buffer_dirty(bh) && !buffer_locked(bh)) {
	reiserfs_free_jh(bh); <--- this could be moved before buffer_locked


if (buffer_locked((journal->j_header_bh))) {
	...
}
journal->j_last_flush_trans_id = trans_id;
journal->j_first_unflushed_offset = offset;
jh = (struct reiserfs_journal_header *)(journal->j_header_bh->b_data); <--- this could be moved before buffer_locked

Mikulas