[PATCH 4.9 70/94] xfs: use ->b_state to fix buffer I/O accounting release race

Greg Kroah-Hartman
2017-06-05T16:26:52Z

From: Brian Foster <bfoster@redhat.com>

commit 63db7c815bc0997c29e484d2409684fdd9fcd93b upstream.

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: 9c7504aa72b6 ("xfs: track and serialize in-flight async buffers against unmount")
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/xfs/xfs_buf.c | 38 ++++++++++++++++++++++++++------------
fs/xfs/xfs_buf.h | 5 ++---
2 files changed, 28 insertions(+), 15 deletions(-)

--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -96,12 +96,16 @@ 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;
- percpu_counter_inc(&bp->b_target->bt_io_count);
+ 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);
}

/*
@@ -109,14 +113,24 @@ xfs_buf_ioacct_inc(
* freed and unaccount from the buftarg.
*/
static inline void
-xfs_buf_ioacct_dec(
+__xfs_buf_ioacct_dec(
struct xfs_buf *bp)
{
- if (!(bp->b_flags & _XBF_IN_FLIGHT))
- return;
+ ASSERT(spin_is_locked(&bp->b_lock));

- bp->b_flags &= ~_XBF_IN_FLIGHT;
- percpu_counter_dec(&bp->b_target->bt_io_count);
+ 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)
+{
+ spin_lock(&bp->b_lock);
+ __xfs_buf_ioacct_dec(bp);
+ spin_unlock(&bp->b_lock);
}

/*
@@ -148,9 +162,9 @@ 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)))
@@ -953,12 +967,12 @@ 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
--- a/fs/xfs/xfs_buf.h
+++ b/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;

@@ -83,14 +82,14 @@ 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" -