Re: [PATCH] btrfs: relocation: fix misplaced barrier in reloc_root_is_dead
From: Qu Wenruo <hidden>
Date: 2021-05-26 06:13:42
Also in:
lkml
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 <redacted> Thanks, Qu
quoted hunk ↗ jump to hunk
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 <redacted> --- 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; } /*