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authorBrian Foster <bfoster@redhat.com>2017-05-31 08:22:52 -0700
committerDarrick J. Wong <darrick.wong@oracle.com>2017-05-31 08:22:52 -0700
commit63db7c815bc0997c29e484d2409684fdd9fcd93b (patch)
tree132764569fad16360e83d774481c35678db5bd0a /fs/xfs/xfs_buf.h
parenta54fba8f5a0dc36161cacdf2aa90f007f702ec1a (diff)
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: 9c7504aa72b6 ("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>
Diffstat (limited to 'fs/xfs/xfs_buf.h')
-rw-r--r--fs/xfs/xfs_buf.h5
1 files changed, 2 insertions, 3 deletions
diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
index 8d1d44f87ce9..1508121f29f2 100644
--- 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;
@@ -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" -