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. Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx> --- v2: - Use ->b_state and ->b_lock instead of custom atomic counter. v1: http://www.spinics.net/lists/linux-xfs/msg06995.html fs/xfs/xfs_buf.c | 38 ++++++++++++++++++++++++++------------ fs/xfs/xfs_buf.h | 5 ++--- 2 files changed, 28 insertions(+), 15 deletions(-) diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c index ba036c1..4e19fda 100644 --- a/fs/xfs/xfs_buf.c +++ b/fs/xfs/xfs_buf.c @@ -97,12 +97,16 @@ 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); } /* @@ -110,14 +114,24 @@ 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); } /* @@ -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 diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h index 8d1d44f..1508121 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" - -- 2.7.4 -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
