Messages in this thread | | | From | Baptiste Lepers <> | Date | Tue, 7 Sep 2021 10:44:17 +1000 | Subject | Re: [PATCH] btrfs: transaction: Fix misplaced barrier in btrfs_record_root_in_trans |
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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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