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

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Date:   Mon, 8 Aug 2022 10:57:45 -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 Mon, 8 Aug 2022, Matthew Wilcox wrote:

> On Mon, Aug 08, 2022 at 10:26:10AM -0400, Mikulas Patocka wrote:
> > On Sun, 7 Aug 2022, Matthew Wilcox wrote:
> > > > +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
> 
> It might be better to think of buffer_locked() as
> buffer_someone_has_exclusive_access().  I can't see the problem with
> moving the reads in reiserfs_free_jh() before the read of 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
> 
> I don't think b_data is going to be changed while someone else holds
> the buffer locked.  That's initialised by set_bh_page(), which is an
> initialisation-time thing, before the BH is visible to any other thread.

So, do you think that we don't need a barrier in buffer_locked()?


There is also this (where the BUG_ON(!buffer_uptodate(bh)) saves it).
                if (buffer_locked(bh)) {
                        int depth;
                        PROC_INFO_INC(sb, scan_bitmap.wait);
                        depth = reiserfs_write_unlock_nested(sb);
                        __wait_on_buffer(bh);
                        reiserfs_write_lock_nested(sb, depth);
                }
                BUG_ON(!buffer_uptodate(bh));
                BUG_ON(atomic_read(&bh->b_count) == 0);

                if (info->free_count == UINT_MAX)
                        reiserfs_cache_bitmap_metadata(sb, bh, info); <--- this could be moved before buffer_locked if there were no BUG_ONs

Mikulas