On 2021/5/26 下午2:09, Baptiste Lepers wrote:
> Fix a misplaced barrier in reloc_root_is_dead. The
> BTRFS_ROOT_DEAD_RELOC_TREE bit should be checked before accessing
> reloc_root.
>
> The fix forces the order documented in the original commit:
> "In the cross section C-D, either caller gets the DEAD bit set, avoiding
> access reloc_root no matter if it's safe or not. Or caller get the DEAD
> bit cleared, then access reloc_root, which is already NULL, nothing will
> be wrong."
>
> static bool reloc_root_is_dead()
> smp_rmb(); <--- misplaced
> if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
> return true;
> return false;
> }
Exactly what I originally purposed to David, we didn't reach an
agreement on where the barrier should be.
At least I'm completely fine with this patch.
Reviewed-by: Qu Wenruo <[email protected]>
Thanks,
Qu
>
> static bool have_reloc_root(struct btrfs_root *root)
> {
> if (reloc_root_is_dead(root))
> return false;
> <--- the barrier should happen here
> if (!root->reloc_root)
> return false;
> return true;
> }
>
> Fixes: 6282675e6708e ("btrfs: relocation: fix reloc_root lifespan and access")
> Signed-off-by: Baptiste Lepers <[email protected]>
> ---
> fs/btrfs/relocation.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index b70be2ac2e9e..8cddcce56bf4 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -289,15 +289,14 @@ static int update_backref_cache(struct btrfs_trans_handle *trans,
>
> static bool reloc_root_is_dead(struct btrfs_root *root)
> {
> + bool is_dead = test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
> /*
> * Pair with set_bit/clear_bit in clean_dirty_subvols and
> * btrfs_update_reloc_root. We need to see the updated bit before
> * trying to access reloc_root
> */
> smp_rmb();
> - if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
> - return true;
> - return false;
> + return is_dead;
> }
>
> /*
>