On Fri, May 04, 2018 at 08:12:47AM -0700, Darrick J. Wong wrote:
> On Fri, May 04, 2018 at 07:31:58AM -0400, Brian Foster wrote:
> > On Thu, May 03, 2018 at 10:53:43AM -0700, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > >
> > > In commit efa092f3d4c6 "[XFS] Fixes a bug in the quota code when
> > > allocating a new dquot record", we allocate a new dquot block, grab a
> > > buffer to initialize it, and return the locked initialized dquot buffer
> > > to the caller for further in-core dquot initialization. Unfortunately,
> > > if the _bmap_finish errored out, _qm_dqalloc would also error out
> > > without bothering to free the (locked) buffer. Leaking a locked buffer
> > > caused hangs in generic/388 when quotas are enabled.
> > >
> > > Furthermore, the _bmap_finish -> _defer_finish conversion in
> > > 310a75a3c6c747 ("xfs: change xfs_bmap_{finish,cancel,init,free} ->
> > > xfs_defer_*") failed to observe that the buffer was held going into
> > > _defer_finish and therefore failed to notice that the buffer lock is
> > > /not/ maintained afterwards. Now that we can bjoin a buffer to a
> > > defer_ops, use this mechanism to ensure that the buffer stays locked
> > > across the _defer_finish. Release the holds and locks on the buffer as
> > > appropriate if we have to error out.
> > >
> > > There is a subtlety here for the caller in that the buffer emerges
> > > locked and held to the transaction, so if the _trans_commit fails we
> > > have to release the buffer explicitly. This fixes the unmount hang
> > > in generic/388 when quotas are enabled.
> > >
> > > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > > ---
> > > fs/xfs/xfs_dquot.c | 48 +++++++++++++++++++++++++++---------------------
> > > 1 file changed, 27 insertions(+), 21 deletions(-)
> > >
> > > diff --git a/fs/xfs/xfs_dquot.c b/fs/xfs/xfs_dquot.c
> > > index a7daef9e16bf..4c39d8632230 100644
> > > --- a/fs/xfs/xfs_dquot.c
> > > +++ b/fs/xfs/xfs_dquot.c
> > > @@ -362,33 +362,39 @@ xfs_qm_dqalloc(
> > > dqp->dq_flags & XFS_DQ_ALLTYPES, bp);
> > >
> > > /*
> > > - * xfs_defer_finish() may commit the current transaction and
> > > - * start a second transaction if the freelist is not empty.
> > > + * Hold the buffer and join it to the dfops so that we'll still own
> > > + * the buffer when we return to the caller. The buffer disposal on
> > > + * error must be paid attention to very carefully, as it has been
> > > + * broken since commit efa092f3d4c6 "[XFS] Fixes a bug in the quota
> > > + * code when allocating a new dquot record" in 2005, and the later
> > > + * conversion to xfs_defer_ops in commit 310a75a3c6c747 failed to keep
> > > + * the buffer locked across the _defer_finish call. We can now do
> > > + * this correctly with xfs_defer_bjoin.
> > > *
> > > - * Since we still want to modify this buffer, we need to
> > > - * ensure that the buffer is not released on commit of
> > > - * the first transaction and ensure the buffer is added to the
> > > - * second transaction.
> > > + * Above, we allocated a disk block for the dquot information and
> > > + * used get_buf to initialize the dquot. If the _defer_bjoin fails,
> > > + * the buffer is still locked to *tpp, so we must _bhold_release and
> > > + * then _trans_brelse the buffer. If the _defer_finish fails, the old
> > > + * transaction is gone but the new buffer is not joined or held to any
> > > + * transaction, so we must _buf_relse it.
> > > *
> > > - * If there is only one transaction then don't stop the buffer
> > > - * from being released when it commits later on.
> > > + * If everything succeeds, the caller of this function is returned a
> > > + * buffer that is locked, held, and joined to the transaction. If the
> > > + * transaction commit fails (in the caller) the caller must unlock the
> > > + * buffer manually.
> >
> > If the buffer is held due to the xfs_defer_bjoin(), doesn't that mean
> > that the caller has to ultimately release it even after successful
> > transaction commit (assuming we don't roll the transaction again
> > somewhere)? I see we have an xfs_trans_brelse() up in xfs_qm_dqread(),
> > but it looks like that only clears the hold if the buffer isn't logged
> > in the tx. Hm?
>
> Correct. The buffer is initialized in the same transaction as the dquot
> block allocation and committed in xfs_defer_finish. After
> initialization (which is to say when we return to xfs_qm_dqtobp), the
> buffer is held, joined, and not logged to the transaction, and nothing
> else is supposed to dirty the buffer. Both buffer and transaction are
> then returned in this state to _dqread, which the in-core dquot state
> out of the dquot buffer and _trans_brelse's the (still clean) buffer,
> which breaks the hold and unlocks the buffer.
>
Ok that makes sense, but doesn't that depend on having a deferred
operation? Is that always guaranteed here?
Brian
> After the refactor we guarantee that the buffer is locked, clean, and
> not attached to a transaction by the time we get to calling
> xfs_dquot_from_disk rather than returning transaction and buffer up the
> call stack and having to reason up the stack about what state they're in.
>
> --D
>
> > Brian
> >
> > > */
> > > -
> > > - xfs_trans_bhold(tp, bp);
> > > -
> > > + xfs_trans_bhold(*tpp, bp);
> > > + error = xfs_defer_bjoin(&dfops, bp);
> > > + if (error) {
> > > + xfs_trans_bhold_release(*tpp, bp);
> > > + xfs_trans_brelse(*tpp, bp);
> > > + goto error1;
> > > + }
> > > error = xfs_defer_finish(tpp, &dfops);
> > > - if (error)
> > > + if (error) {
> > > + xfs_buf_relse(bp);
> > > goto error1;
> > > -
> > > - /* Transaction was committed? */
> > > - if (*tpp != tp) {
> > > - tp = *tpp;
> > > - xfs_trans_bjoin(tp, bp);
> > > - } else {
> > > - xfs_trans_bhold_release(tp, bp);
> > > }
> > > -
> > > - *O_bpp = bp;
> > > return 0;
> > >
> > > error1:
> > > --
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