btrfs: fix root-in-trans fast-path ordering

btrfs_record_root_in_trans() has a lockless fast path for shareable
roots. It skips reloc_mutex when root->last_trans matches the current
transaction and BTRFS_ROOT_IN_TRANS_SETUP is clear.

The writer side publishes that state in two phases: it sets
IN_TRANS_SETUP before updating root->last_trans, then clears the bit
after btrfs_init_reloc_root() finishes. However, the reader-side
smp_rmb() is before both loads, so it does not order the last_trans load
against the later bit test. A reader can observe the new last_trans value
while missing the setup bit and return before the relocation-root setup
is complete.

Read root->last_trans first, then issue the read barrier before testing
IN_TRANS_SETUP. Also use clear_bit_unlock() for the writer's final clear
and test_bit_acquire() for the successful fast path, so the lockless
return observes the setup done before the bit was cleared.

Fixes: 7585717f304f ("Btrfs: fix relocation races")
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

0 files changed, 0 insertions, 0 deletions