xfs: use ->b_state to fix buffer I/O accounting release race

We've had user reports of unmount hangs in xfs_wait_buftarg() that
analysis shows is due to btp->bt_io_count == -1. bt_io_count
represents the count of in-flight asynchronous buffers and thus
should always be >= 0. xfs_wait_buftarg() waits for this value to
stabilize to zero in order to ensure that all untracked (with
respect to the lru) buffers have completed I/O processing before
unmount proceeds to tear down in-core data structures.

The value of -1 implies an I/O accounting decrement race. Indeed,
the fact that xfs_buf_ioacct_dec() is called from xfs_buf_rele()
(where the buffer lock is no longer held) means that bp->b_flags can
be updated from an unsafe context. While a user-level reproducer is
currently not available, some intrusive hacks to run racing buffer
lookups/ioacct/releases from multiple threads was used to
successfully manufacture this problem.

Existing callers do not expect to acquire the buffer lock from
xfs_buf_rele(). Therefore, we can not safely update ->b_flags from
this context. It turns out that we already have separate buffer
state bits and associated serialization for dealing with buffer LRU
state in the form of ->b_state and ->b_lock. Therefore, replace the
_XBF_IN_FLIGHT flag with a ->b_state variant, update the I/O
accounting wrappers appropriately and make sure they are used with
the correct locking. This ensures that buffer in-flight state can be
modified at buffer release time without racing with modifications
from a buffer lock holder.

Fixes: 9c7504aa72 ("xfs: track and serialize in-flight async buffers against unmount")
Cc: <stable@vger.kernel.org> # v4.8+
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Tested-by: Libor Pechacek <lpechacek@suse.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_buf.c

@@ -97,27 +97,41 @@ static inline void
 xfs_buf_ioacct_inc(
 	struct xfs_buf	*bp)
 {
-	if (bp->b_flags & (XBF_NO_IOACCT|_XBF_IN_FLIGHT))
+	if (bp->b_flags & XBF_NO_IOACCT)
 		return;
 
 	ASSERT(bp->b_flags & XBF_ASYNC);
-	bp->b_flags |= _XBF_IN_FLIGHT;
+	spin_lock(&bp->b_lock);
+	if (!(bp->b_state & XFS_BSTATE_IN_FLIGHT)) {
+		bp->b_state |= XFS_BSTATE_IN_FLIGHT;
 		percpu_counter_inc(&bp->b_target->bt_io_count);
 	}
+	spin_unlock(&bp->b_lock);
+}
 
 /*
  * Clear the in-flight state on a buffer about to be released to the LRU or
  * freed and unaccount from the buftarg.
  */
+static inline void
+__xfs_buf_ioacct_dec(
+	struct xfs_buf	*bp)
+{
+	ASSERT(spin_is_locked(&bp->b_lock));
+
+	if (bp->b_state & XFS_BSTATE_IN_FLIGHT) {
+		bp->b_state &= ~XFS_BSTATE_IN_FLIGHT;
+		percpu_counter_dec(&bp->b_target->bt_io_count);
+	}
+}
+
 static inline void
 xfs_buf_ioacct_dec(
 	struct xfs_buf	*bp)
 {
-	if (!(bp->b_flags & _XBF_IN_FLIGHT))
-		return;
-
-	bp->b_flags &= ~_XBF_IN_FLIGHT;
-	percpu_counter_dec(&bp->b_target->bt_io_count);
+	spin_lock(&bp->b_lock);
+	__xfs_buf_ioacct_dec(bp);
+	spin_unlock(&bp->b_lock);
 }
 
 /*
@@ -149,9 +163,9 @@ xfs_buf_stale(
 	 * unaccounted (released to LRU) before that occurs. Drop in-flight
 	 * status now to preserve accounting consistency.
 	 */
-	xfs_buf_ioacct_dec(bp);
-
 	spin_lock(&bp->b_lock);
+	__xfs_buf_ioacct_dec(bp);
+
 	atomic_set(&bp->b_lru_ref, 0);
 	if (!(bp->b_state & XFS_BSTATE_DISPOSE) &&
 	    (list_lru_del(&bp->b_target->bt_lru, &bp->b_lru)))
@@ -979,12 +993,12 @@ xfs_buf_rele(
 		 * ensures the decrement occurs only once per-buf.
 		 */
 		if ((atomic_read(&bp->b_hold) == 1) && !list_empty(&bp->b_lru))
-			xfs_buf_ioacct_dec(bp);
+			__xfs_buf_ioacct_dec(bp);
 		goto out_unlock;
 	}
 
 	/* the last reference has been dropped ... */
-	xfs_buf_ioacct_dec(bp);
+	__xfs_buf_ioacct_dec(bp);
 	if (!(bp->b_flags & XBF_STALE) && atomic_read(&bp->b_lru_ref)) {
 		/*
 		 * If the buffer is added to the LRU take a new reference to the

fs/xfs/xfs_buf.h

@@ -63,7 +63,6 @@ typedef enum {
 #define _XBF_KMEM	 (1 << 21)/* backed by heap memory */
 #define _XBF_DELWRI_Q	 (1 << 22)/* buffer on a delwri queue */
 #define _XBF_COMPOUND	 (1 << 23)/* compound buffer */
-#define _XBF_IN_FLIGHT	 (1 << 25) /* I/O in flight, for accounting purposes */
 
 typedef unsigned int xfs_buf_flags_t;
 
@@ -84,14 +83,14 @@ typedef unsigned int xfs_buf_flags_t;
 	{ _XBF_PAGES,		"PAGES" }, \
 	{ _XBF_KMEM,		"KMEM" }, \
 	{ _XBF_DELWRI_Q,	"DELWRI_Q" }, \
-	{ _XBF_COMPOUND,	"COMPOUND" }, \
-	{ _XBF_IN_FLIGHT,	"IN_FLIGHT" }
+	{ _XBF_COMPOUND,	"COMPOUND" }
 
 
 /*
  * Internal state flags.
  */
 #define XFS_BSTATE_DISPOSE	 (1 << 0)	/* buffer being discarded */
+#define XFS_BSTATE_IN_FLIGHT	 (1 << 1)	/* I/O in flight */
 
 /*
  * The xfs_buftarg contains 2 notions of "sector size" -