Re: [PATCH 1/2] xfs: always rejoin held resources during defer roll

Darrick J. Wong
2019-04-25T15:54:50Z

On Thu, Apr 25, 2019 at 08:06:29AM -0700, Darrick J. Wong wrote:
> On Thu, Apr 25, 2019 at 07:16:50AM -0400, Brian Foster wrote:
> > On Wed, Apr 24, 2019 at 10:47:47PM -0700, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@oracle.com>
> > >
> > > During testing of xfs/141 on a V4 filesystem, I observed some
> > > inconsistent behavior with regards to resources that are held (i.e.
> > > remain locked) across a defer roll. The transaction roll always gives
> > > the defer roll function a new transaction, even if committing the old
> > > transaction fails. However, the defer roll function only rejoins the
> > > held resources if the transaction commit succeedied. This means that
> > > callers of defer roll have to figure out whether the held resources are
> > > attached to the transaction being passed back.
> > >
> > > Worse yet, if the defer roll was part of a defer finish call, we have a
> > > third possibility: the defer finish could pass back a dirty transaction
> > > with dirty held resources and an error code.
> > >
> > > The only sane way to handle all of these scenarios is to require that
> > > the code that held the resource either cancel the transaction before
> > > unlocking and releasing the resources, or use functions that detach
> > > resources from a transaction properly (e.g. xfs_trans_brelse) if they
> > > need to drop the reference before committing or cancelling the
> > > transaction.
> > >
> > > In order to make this so, change the defer roll code to join held
> > > resources to the new transaction unconditionally and fix all the bhold
> > > callers to release the held buffers correctly.
> > >
> > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > > ---
> > > fs/xfs/libxfs/xfs_attr.c | 31 ++++++++++---------------------
> > > fs/xfs/libxfs/xfs_attr.h | 2 +-
> > > fs/xfs/libxfs/xfs_defer.c | 14 +++++++++-----
> > > fs/xfs/xfs_dquot.c | 22 +++++++++++-----------
> > > 4 files changed, 31 insertions(+), 38 deletions(-)
> > >
> > > diff --git a/fs/xfs/libxfs/xfs_attr.c b/fs/xfs/libxfs/xfs_attr.c
> > > index 2dd9ee2a2e08..ad954138243e 100644
> > > --- a/fs/xfs/libxfs/xfs_attr.c
> > > +++ b/fs/xfs/libxfs/xfs_attr.c
> > > @@ -224,10 +224,10 @@ xfs_attr_try_sf_addname(
> > > */
> > > int
> > > xfs_attr_set_args(
> > > - struct xfs_da_args *args,
> > > - struct xfs_buf **leaf_bp)
> > > + struct xfs_da_args *args)
> > > {
> > > struct xfs_inode *dp = args->dp;
> > > + struct xfs_buf *leaf_bp = NULL;
> > > int error;
> > >
> > > /*
> > > @@ -255,7 +255,7 @@ xfs_attr_set_args(
> > > * It won't fit in the shortform, transform to a leaf block.
> > > * GROT: another possible req'mt for a double-split btree op.
> > > */
> > > - error = xfs_attr_shortform_to_leaf(args, leaf_bp);
> > > + error = xfs_attr_shortform_to_leaf(args, &leaf_bp);
> > > if (error)
> > > return error;
> > >
> > > @@ -264,22 +264,15 @@ xfs_attr_set_args(
> > > * concurrent AIL push cannot grab the half-baked leaf
> > > * buffer and run into problems with the write verifier.
> > > */
> > > - xfs_trans_bhold(args->trans, *leaf_bp);
> > > + xfs_trans_bhold(args->trans, leaf_bp);
> > >
> > > error = xfs_defer_finish(&args->trans);
> > > - if (error)
> > > + if (error) {
> > > + xfs_trans_brelse(args->trans, leaf_bp);
> > > return error;
> > > + }
> >
> > Shouldn't we just unconditionally release the bhold here? Then we cover
> > the scenario where the buffer might somehow be dirty.
>
> Doh. Yeah, I missed that; thanks for catching it.
>
> > It also seems more
> > logically consistent since this code is now responsible only for the
> > bhold (to keep the buf locked and deal with the AIL writeback verifier
> > race thing). Beyond that, the final transaction can release the buffer
> > on commit or abort. E.g., something like the following..?
> >
> > /*
> > * Hold the buffer locked to prevent ... Release the hold to
> > * transfer the buf to the final tx.
> > */
> > xfs_trans_bhold(args->trans, leaf_bp);
> > error = xfs_defer_finish(&args->trans);
> > xfs_trans_bhold_release(args->trans, leaf_bp);
> > if (error)
> > return error;
> >
> > BTW out of curiosity, did you happen to identify what deferred operation
> > was actually queued here that triggered a tx roll in your xfs/141 test?
> > Was it an allocation/agfl fixup or something?
>
> I only looked after the fact but it looked like it was an agfl fixup.
> The fs was a V4 filesystem, fwiw.
>
> > >
> > > - /*
> > > - * Commit the leaf transformation. We'll need another
> > > - * (linked) transaction to add the new attribute to the
> > > - * leaf.
> > > - */
> > > - error = xfs_trans_roll_inode(&args->trans, dp);
> > > - if (error)
> > > - return error;
> > > - xfs_trans_bjoin(args->trans, *leaf_bp);
> > > - *leaf_bp = NULL;
> > > + xfs_trans_bhold_release(args->trans, leaf_bp);
> > > }
> > >
> > > if (xfs_bmap_one_block(dp, XFS_ATTR_FORK))
> > > @@ -322,7 +315,6 @@ xfs_attr_set(
> > > int flags)
> > > {
> > > struct xfs_mount *mp = dp->i_mount;
> > > - struct xfs_buf *leaf_bp = NULL;
> > > struct xfs_da_args args;
> > > struct xfs_trans_res tres;
> > > int rsvd = (flags & ATTR_ROOT) != 0;
> > > @@ -381,9 +373,9 @@ xfs_attr_set(
> > > goto out_trans_cancel;
> > >
> > > xfs_trans_ijoin(args.trans, dp, 0);
> > > - error = xfs_attr_set_args(&args, &leaf_bp);
> > > + error = xfs_attr_set_args(&args);
> > > if (error)
> > > - goto out_release_leaf;
> > > + goto out_trans_cancel;
> > > if (!args.trans) {
> > > /* shortform attribute has already been committed */
> > > goto out_unlock;
> > > @@ -408,9 +400,6 @@ xfs_attr_set(
> > > xfs_iunlock(dp, XFS_ILOCK_EXCL);
> > > return error;
> > >
> > > -out_release_leaf:
> > > - if (leaf_bp)
> > > - xfs_trans_brelse(args.trans, leaf_bp);
> > > out_trans_cancel:
> > > if (args.trans)
> > > xfs_trans_cancel(args.trans);
> > > diff --git a/fs/xfs/libxfs/xfs_attr.h b/fs/xfs/libxfs/xfs_attr.h
> > > index 2297d8467666..3b0dce06e454 100644
> > > --- a/fs/xfs/libxfs/xfs_attr.h
> > > +++ b/fs/xfs/libxfs/xfs_attr.h
> > > @@ -140,7 +140,7 @@ int xfs_attr_get(struct xfs_inode *ip, const unsigned char *name,
> > > unsigned char *value, int *valuelenp, int flags);
> > > int xfs_attr_set(struct xfs_inode *dp, const unsigned char *name,
> > > unsigned char *value, int valuelen, int flags);
> > > -int xfs_attr_set_args(struct xfs_da_args *args, struct xfs_buf **leaf_bp);
> > > +int xfs_attr_set_args(struct xfs_da_args *args);
> > > int xfs_attr_remove(struct xfs_inode *dp, const unsigned char *name, int flags);
> > > int xfs_attr_remove_args(struct xfs_da_args *args);
> > > int xfs_attr_list(struct xfs_inode *dp, char *buffer, int bufsize,
> > > diff --git a/fs/xfs/libxfs/xfs_defer.c b/fs/xfs/libxfs/xfs_defer.c
> > > index 94f00427de98..1c6bf2105939 100644
> > > --- a/fs/xfs/libxfs/xfs_defer.c
> > > +++ b/fs/xfs/libxfs/xfs_defer.c
> > > @@ -274,13 +274,15 @@ xfs_defer_trans_roll(
> > >
> > > trace_xfs_defer_trans_roll(tp, _RET_IP_);
> > >
> > > - /* Roll the transaction. */
> > > + /*
> > > + * Roll the transaction. Rolling always given a new transaction (even
> > > + * if committing the old one fails!) to hand back to the caller, so we
> > > + * join the held resources to the new transaction so that we always
> > > + * return with the held resources joined to @tpp, no matter what
> > > + * happened.
> > > + */
> > > error = xfs_trans_roll(tpp);
> > > tp = *tpp;
> > > - if (error) {
> > > - trace_xfs_defer_trans_roll_error(tp, error);
> > > - return error;
> > > - }
> > >
> > > /* Rejoin the joined inodes. */
> > > for (i = 0; i < ipcount; i++)
> > > @@ -292,6 +294,8 @@ xfs_defer_trans_roll(
> > > xfs_trans_bhold(tp, bplist[i]);
> > > }
> > >
> > > + if (error)
> > > + trace_xfs_defer_trans_roll_error(tp, error);
> > > return error;
> > > }
> > >
> > > diff --git a/fs/xfs/xfs_dquot.c b/fs/xfs/xfs_dquot.c
> > > index 87e6dd5326d5..7f058120a921 100644
> > > --- a/fs/xfs/xfs_dquot.c
> > > +++ b/fs/xfs/xfs_dquot.c
> > > @@ -359,9 +359,8 @@ xfs_dquot_disk_alloc(
> > > */
> > > xfs_trans_bhold(tp, bp);
> > > error = xfs_defer_finish(tpp);
> > > - tp = *tpp;
> > > if (error) {
> > > - xfs_buf_relse(bp);
> > > + xfs_trans_brelse(*tpp, bp);
> > > return error;
> > > }
> >
> > Similar comment here wrt to the dirty buf case. This code means that if
> > the buffer remained dirty, we'd hold it to the tx, brelse will skip it
> > and we wouldn't have set *bpp before we return the error. The caller
> > cancels the tx, but still owns the held buf yet doesn't have a pointer
> > to it.
> >
> > The dirty buffer scenario may not play out in either of these contexts,
>
> It won't until a security researcher creates a specially crafted corrupt
> filesystem etc. :)
>
> > but I'd prefer to try and use/propagate error handling patterns that are
> > totally robust if at all possible. We'll eventually have that scenario
> > play out somewhere I'm sure and chances are this code will be
> > reused/copied/referenced for any new bhold+dfops situations. Thinking
> > about it, all it would take is some new pre-roll error handling down in
> > xfs_defer_finish() to quietly turn these error checks into a leak
> > vector.
>
> Sounds like a good place to stuff in another error injector...
>
> > This case looks like it actually expects the buffer to remain held on
> > return so perhaps we just need to set the *bpp pointer sooner. The
> > caller can then commit the final tx and set it's *bpp (if the commit is
> > successful) or release bp itself and cancel the tx if this function
> > returned an error.
>
> Hm. xfs_dquot_disk_alloc passes the dquot buffer pointer a couple of
> levels up (to xfs_qm_dqread) so that we can fill the incore dquot with
> whatever's written on disk, so I think we can stick to returning 0 with
> the buffer pointer set, or returning an error and no buffer at all.
> I see two advantages of that: first, both hold-defer-unhold cases have
> exactly the same code, and we can simplify the error handling in
> xfs_qm_dqread_alloc.

I am totally wrong about this, because the attr leaf and dquot cases are
very different. For the attr leaf we want to hold the buffer locked
across a roll so we can add the attr to the new leaf buffer and commit
it. For the dquot case we want to hold the buffer locked past the end
of the transaction commit so that we can use its data to populate the
incore dquot. This means we can't unconditionally release the bhold
here -- we only want this when errors happen and we're trying to get rid
of our reference to the buffer while backing out.

--D

>
> > > *bpp = bp;
> > > @@ -521,7 +520,7 @@ xfs_qm_dqread_alloc(
> > > struct xfs_buf **bpp)
> > > {
> > > struct xfs_trans *tp;
> > > - struct xfs_buf *bp;
> > > + struct xfs_buf *bp = NULL;
> > > int error;
> > >
> > > error = xfs_trans_alloc(mp, &M_RES(mp)->tr_qm_dqalloc,
> > > @@ -529,24 +528,25 @@ xfs_qm_dqread_alloc(
> > > if (error)
> > > goto err;
> > >
> > > + /*
> > > + * This function returns with the buffer held to the transaction, so
> > > + * either we pass it back to our caller still locked or cancel the
> > > + * transaction and unlock it manually if we're not passing it back.
> > > + */
> > > error = xfs_dquot_disk_alloc(&tp, dqp, &bp);
> > > if (error)
> > > goto err_cancel;
> > >
> > > error = xfs_trans_commit(tp);
> > > - if (error) {
> > > - /*
> > > - * Buffer was held to the transaction, so we have to unlock it
> > > - * manually here because we're not passing it back.
> > > - */
> > > - xfs_buf_relse(bp);
> > > - goto err;
> > > - }
> > > + if (error)
> > > + goto err_cancel;
> >
> > A failed xfs_trans_commit() is equivalent to an xfs_trans_cancel() (it
> > frees the tx) so there's no need to call it here.
>
> Will fix.
>
> --D
>
> > Brian
> >
> > > *bpp = bp;
> > > return 0;
> > >
> > > err_cancel:
> > > xfs_trans_cancel(tp);
> > > + if (bp)
> > > + xfs_buf_relse(bp);
> > > err:
> > > return error;
> > > }