Re: [PATCH v5] add barriers to buffer functions

[Matthew Wilcox]

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.