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
