On 30.05.2017 16:22, 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. > > Fixes: 9c7504aa72b6 ("xfs: track and serialize in-flight async buffers against unmount") > CC: stable@xxxxxxxxxxxxxxx # v4.8+ > Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx> > Reviewed-by: Nikolay Borisov <nborisov@xxxxxxxx> Our QA guys managed to reliably reproduce this on an HP GEN8 box, running test_dbench-4_0_async-scenario-run (I don't know if it's some internal flavor of dbench). With this patch applied the issue is gone. You can add: Tested-by: Libor Pechacek <lpechacek@xxxxxxxx> > --- > > v3: > - Added Fixes/Reviewed-by tags and CC stable. > v2: http://www.spinics.net/lists/linux-xfs/msg07062.html > - 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" - >