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

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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: 7585717f30 ("Btrfs: fix relocation races")
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Cen Zhang
2026-07-31 13:30:43 +02:00
committed by David Sterba
parent 622808498b
commit 5004bcdb4f
1 changed files with 10 additions and 8 deletions
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fs/btrfs/transaction.c
+10 -8
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@@ -456,12 +456,12 @@ static int record_root_in_trans(struct btrfs_trans_handle *trans,
*
* When this is zero, they can trust root->last_trans and fly
* through btrfs_record_root_in_trans without having to take the
* lock. smp_wmb() makes sure that all the writes above are
* done before we pop in the zero below
* lock. smp_wmb() makes sure readers that see the last_trans
* update also see IN_TRANS_SETUP set, and clear_bit_unlock()
* publishes the relocation setup before we clear the bit.
*/
ret = btrfs_init_reloc_root(trans, root);
smp_mb__before_atomic();
clear_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state);
clear_bit_unlock(BTRFS_ROOT_IN_TRANS_SETUP, &root->state);
}
return ret;
}
@@ -499,10 +499,12 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
* see record_root_in_trans for comments about IN_TRANS_SETUP usage
* and barriers
*/
smp_rmb();
if (btrfs_get_root_last_trans(root) == trans->transid &&
!test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))
return 0;
if (btrfs_get_root_last_trans(root) == trans->transid) {
/* Order the last_trans load before testing IN_TRANS_SETUP. */
smp_rmb();
if (!test_bit_acquire(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))
return 0;
}
mutex_lock(&fs_info->reloc_mutex);
ret = record_root_in_trans(trans, root, false);