On Tue, May 08, 2012 at 03:57:39PM -0700, Mark Fasheh wrote:
> Hi Jan, comments inline as usual!
>
> > This function must not call free_extent_buffer(eb) in line 1306 after
> > applying your patch set (immediately before the break). Second, I think
> > we'd better add a blocking read lock on eb after incrementing it's
> > refcount, because we need the current content to stay as it is. Both
> > isn't part of your patches, but it might be easier if you make that
> > bugfix change as a 3/4 patch within your set and turn this one into 4/4.
> > If you don't like that, I'll send a separate patch for it. Don't miss
> > the unlock if you do it ;-)
>
> Ok, I think I was able to figure out and add the correct locking calls.
>
> Basically I believe I need to wrap access around:
>
> btrfs_tree_read_lock(eb);
> btrfs_set_lock_blocking_rw(eb, BTRFS_READ_LOCK);
>
> <read eb contents>
>
> btrfs_tree_read_unlock_blocking(eb);
You only need a blocking lock if you're scheduling. Otherwise the
spinlock variant is fine.
> > > +
> > > + while (1) {
> > > + ret = btrfs_find_one_extref(fs_root, inum, offset, path, &iref2,
> > > + &offset);
> > > + if (ret < 0)
> > > + break;
> > > + if (ret) {
> > > + ret = found ? 0 : -ENOENT;
> > > + break;
> > > + }
> > > + ++found;
> > > +
> > > + slot = path->slots[0];
> > > + eb = path->nodes[0];
> > > + /* make sure we can use eb after releasing the path */
> > > + atomic_inc(&eb->refs);
> >
> > You need a blocking read lock here, too. Grab it before releasing the path.
If you're calling btrfs_search_slot, it will give you a blocking lock
on the leaf. If you set path->leave_spinning before the call, you'll
have a spinning lock on the leaf.
If you unlock a block that you got from a path (like eb =
path->nodes[0]), the path structure has a flag for each level that
indicates if that block was locked or not. See btrfs_release_path().
So, don't fiddle the locks without fiddling the paths.
You can switch from spinning to/from blocking without touching the path, it
figures that out.
-chris
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