On Wed, May 09, 2012 at 19:02 (+0200), Chris Mason wrote:
>>>> +
>>>> + 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.
Note that we're releasing the path shortly after. My suggestion was to
grab an *additional* read lock after atomic_inc(&eb->refs) (probably
better extent_buffer_get(eb)) and before btrfs_path_release().
An alternative would be to set path->locks[0] to NULL, which would
btrfs_release_path prevent from unlocking it, kidnapping its lock for
our purpose. But I much prefer the open coded solution.
-Jan
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