On 24.05.2017 17:45, Brian Foster wrote:
> 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>
Perhaps a Fixes: 9c7504aa72b6 ("xfs: track and serialize in-flight async
buffers against unmount") tag is in order.
Also I believe it warrants a CC: stable@xxxxxxxxxxxxxxx #4.9
> ---
>
> 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" -
>
Reviewed-by: Nikolay Borisov <nborisov@xxxxxxxx>
--
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
