* [PATCH] btrfs: relocation: fix misplaced barrier in reloc_root_is_dead
@ 2021-05-26 6:09 Baptiste Lepers
2021-05-26 6:13 ` Qu Wenruo
0 siblings, 1 reply; 2+ messages in thread
From: Baptiste Lepers @ 2021-05-26 6:09 UTC (permalink / raw)
Cc: Baptiste Lepers, Chris Mason, Josef Bacik, David Sterba,
Qu Wenruo, linux-btrfs, linux-kernel
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
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] btrfs: relocation: fix misplaced barrier in reloc_root_is_dead
2021-05-26 6:09 [PATCH] btrfs: relocation: fix misplaced barrier in reloc_root_is_dead Baptiste Lepers
@ 2021-05-26 6:13 ` Qu Wenruo
0 siblings, 0 replies; 2+ messages in thread
From: Qu Wenruo @ 2021-05-26 6:13 UTC (permalink / raw)
To: Baptiste Lepers
Cc: Chris Mason, Josef Bacik, David Sterba, Qu Wenruo, linux-btrfs,
linux-kernel
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;
> }
>
> /*
>
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