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SubjectRe: [PATCH] btrfs: transaction: Fix misplaced barrier in btrfs_record_root_in_trans
No, they need to be between the reads to have an effect. See
https://www.kernel.org/doc/Documentation/memory-barriers.txt §SMP
BARRIER PAIRING ("When dealing with CPU-CPU interactions..."). You
will see that the barriers are always between the ordered reads and
not before.

I think that Paul, the barrier guru, can confirm that the barrier was
misplaced in the original code? :)


On Tue, Sep 7, 2021 at 10:43 AM Baptiste Lepers
<baptiste.lepers@gmail.com> wrote:
>
>
>
> On Mon, Sep 6, 2021 at 10:27 PM David Sterba <dsterba@suse.cz> wrote:
>>
>> On Mon, Sep 06, 2021 at 11:25:59AM +1000, Baptiste Lepers wrote:
>> > Per comment, record_root_in_trans orders the writes of the root->state
>> > and root->last_trans:
>> > set_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state);
>> > smp_wmb();
>> > root->last_trans = trans->transid;
>> >
>> > But the barrier that enforces the order on the read side is misplaced:
>> > smp_rmb(); <-- misplaced
>> > if (root->last_trans == trans->transid &&
>> > <-- missing barrier here -->
>> > !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))
>> >
>> > This patches fixes the ordering and wraps the racy accesses with
>> > READ_ONCE and WRITE_ONCE calls to avoid load/store tearing.
>> >
>> > Fixes: 7585717f304f5 ("Btrfs: fix relocation races")
>> > Signed-off-by: Baptiste Lepers <baptiste.lepers@gmail.com>
>> > ---
>> > fs/btrfs/transaction.c | 7 ++++---
>> > 1 file changed, 4 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
>> > index 14b9fdc8aaa9..a609222e6704 100644
>> > --- a/fs/btrfs/transaction.c
>> > +++ b/fs/btrfs/transaction.c
>> > @@ -437,7 +437,7 @@ static int record_root_in_trans(struct btrfs_trans_handle *trans,
>> > (unsigned long)root->root_key.objectid,
>> > BTRFS_ROOT_TRANS_TAG);
>> > spin_unlock(&fs_info->fs_roots_radix_lock);
>> > - root->last_trans = trans->transid;
>> > + WRITE_ONCE(root->last_trans, trans->transid);
>> >
>> > /* this is pretty tricky. We don't want to
>> > * take the relocation lock in btrfs_record_root_in_trans
>> > @@ -489,7 +489,7 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
>> > struct btrfs_root *root)
>> > {
>> > struct btrfs_fs_info *fs_info = root->fs_info;
>> > - int ret;
>> > + int ret, last_trans;
>> >
>> > if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
>> > return 0;
>> > @@ -498,8 +498,9 @@ 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
>> > */
>> > + last_trans = READ_ONCE(root->last_trans);
>> > smp_rmb();
>> > - if (root->last_trans == trans->transid &&
>> > + if (last_trans == trans->transid &&
>> > !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))
>>
>> Aren't the smp_rmb barriers supposed to be used before the condition?
>
>
> No, they need to be between the reads to have an effect. See https://www.kernel.org/doc/Documentation/memory-barriers.txt §SMP BARRIER PAIRING ("When dealing with CPU-CPU interactions..."). You will see that the barriers are always between the ordered reads and not before.
>
> I think that Paul, the barrier guru, can confirm that the barrier was misplaced in the original code? :)

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