[PATCH] xfs: annotate lockless bli_flags access in buf item paths

[Cen Zhang]

xfs_buf_item_unpin() and xfs_buf_item_committed() read bip->bli_flags
without holding the buffer lock, while xfs_buf_item_release() clears
XFS_BLI_LOGGED, XFS_BLI_HOLD, and XFS_BLI_ORDERED under that lock.
This can happen when an older checkpoint still holds a CIL reference to
the BLI while a new transaction is finishing with the buffer.

The lockless readers only test XFS_BLI_STALE and
XFS_BLI_INODE_ALLOC_BUF, which are disjoint from the bits being
cleared, so correctness is not affected in practice.  However, the
plain C accesses still constitute a data race that may allow the
compiler to optimize them in unexpected ways (e.g., load tearing or
fused reloads), so they should be marked explicitly.

Annotate the two lockless reads with READ_ONCE(), make the clearing
store in xfs_buf_item_release() a WRITE_ONCE(), and use READ_ONCE() in
the buf-item tracepoint that may snapshot bli_flags without the lock.

Fixes: 8e1238508633 ("xfs: remove stale parameter from ->iop_unpin method")
Fixes: 71e330b59390 ("xfs: Introduce delayed logging core code")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Cen Zhang <zzzccc427@xxxxxxxxx>
---
 fs/xfs/xfs_buf_item.c | 18 +++++++++++-------
 fs/xfs/xfs_trace.h    |  2 +-
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
index 8487635579e5..c3d0dc17ee10 100644
--- a/fs/xfs/xfs_buf_item.c
+++ b/fs/xfs/xfs_buf_item.c
@@ -502,7 +502,8 @@ xfs_buf_item_unpin(
 {
 	struct xfs_buf_log_item	*bip = BUF_ITEM(lip);
 	struct xfs_buf		*bp = bip->bli_buf;
-	int			stale = bip->bli_flags & XFS_BLI_STALE;
+	unsigned int		flags = READ_ONCE(bip->bli_flags);
+	int			stale = flags & XFS_BLI_STALE;
 	int			freed;
 
 	ASSERT(bp->b_log_item == bip);
@@ -679,13 +680,14 @@ xfs_buf_item_release(
 {
 	struct xfs_buf_log_item	*bip = BUF_ITEM(lip);
 	struct xfs_buf		*bp = bip->bli_buf;
-	bool			hold = bip->bli_flags & XFS_BLI_HOLD;
-	bool			stale = bip->bli_flags & XFS_BLI_STALE;
+	unsigned int		flags = bip->bli_flags;
+	bool			hold = flags & XFS_BLI_HOLD;
+	bool			stale = flags & XFS_BLI_STALE;
 	bool			aborted = test_bit(XFS_LI_ABORTED,
 						   &lip->li_flags);
-	bool			dirty = bip->bli_flags & XFS_BLI_DIRTY;
+	bool			dirty = flags & XFS_BLI_DIRTY;
 #if defined(DEBUG) || defined(XFS_WARN)
-	bool			ordered = bip->bli_flags & XFS_BLI_ORDERED;
+	bool			ordered = flags & XFS_BLI_ORDERED;
 #endif
 
 	trace_xfs_buf_item_release(bip);
@@ -705,7 +707,8 @@ xfs_buf_item_release(
 	 * per-transaction state from the bli, which has been copied above.
 	 */
 	bp->b_transp = NULL;
-	bip->bli_flags &= ~(XFS_BLI_LOGGED | XFS_BLI_HOLD | XFS_BLI_ORDERED);
+	WRITE_ONCE(bip->bli_flags,
+		   flags & ~(XFS_BLI_LOGGED | XFS_BLI_HOLD | XFS_BLI_ORDERED));
 
 	/* If there are other references, then we have nothing to do. */
 	if (!atomic_dec_and_test(&bip->bli_refcount))
@@ -792,10 +795,11 @@ xfs_buf_item_committed(
 	xfs_lsn_t		lsn)
 {
 	struct xfs_buf_log_item	*bip = BUF_ITEM(lip);
+	unsigned int		flags = READ_ONCE(bip->bli_flags);
 
 	trace_xfs_buf_item_committed(bip);
 
-	if ((bip->bli_flags & XFS_BLI_INODE_ALLOC_BUF) && lip->li_lsn != 0)
+	if ((flags & XFS_BLI_INODE_ALLOC_BUF) && lip->li_lsn != 0)
 		return lip->li_lsn;
 	return lsn;
 }
diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
index 813e5a9f57eb..2069534bb0c1 100644
--- a/fs/xfs/xfs_trace.h
+++ b/fs/xfs/xfs_trace.h
@@ -895,7 +895,7 @@ DECLARE_EVENT_CLASS(xfs_buf_item_class,
 	),
 	TP_fast_assign(
 		__entry->dev = bip->bli_buf->b_target->bt_dev;
-		__entry->bli_flags = bip->bli_flags;
+		__entry->bli_flags = READ_ONCE(bip->bli_flags);
 		__entry->bli_recur = bip->bli_recur;
 		__entry->bli_refcount = atomic_read(&bip->bli_refcount);
 		__entry->buf_bno = xfs_buf_daddr(bip->bli_buf);
-- 
2.34.1