[PATCH] btrfs: relocation: fix misplaced barrier in reloc_root_is_dead

[PATCH] btrfs: relocation: fix misplaced barrier in reloc_root_is_dead

[Baptiste Lepers]

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;
}

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 <baptiste.lepers@gmail.com>
---
 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;
 }
 
 /*
-- 
2.17.1

Re: [PATCH] btrfs: relocation: fix misplaced barrier in reloc_root_is_dead

[Qu Wenruo]

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 <wqu@suse.com>

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 <baptiste.lepers@gmail.com>
> ---
>   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;
>   }
>
>   /*
>