Re: [PATCH] btrfs: transaction: Fix misplaced barrier in btrfs_record_root_in_trans

[Baptiste Lepers]

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@xxxxxxxxx> wrote:
>
>
>
> On Mon, Sep 6, 2021 at 10:27 PM David Sterba <dsterba@xxxxxxx> 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@xxxxxxxxx>
>> > ---
>> >  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? :)