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xfs: use ->b_state to fix buffer I/O accounting release race

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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: 9c7504aa72 ("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>
This commit is contained in:
Brian Foster 2017-05-31 08:22:52 -07:00 committed by Darrick J. Wong
parent a54fba8f5a
commit 63db7c815b
2 changed files with 28 additions and 15 deletions
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38
fs/xfs/xfs_buf.c
View File

@ -97,27 +97,41 @@ 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);
}
/*
* Clear the in-flight state on a buffer about to be released to the LRU or
* freed and unaccount from the buftarg.
*/
static inline void
__xfs_buf_ioacct_dec(
struct xfs_buf *bp)
{
ASSERT(spin_is_locked(&bp->b_lock));
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)
{
if (!(bp->b_flags & _XBF_IN_FLIGHT))
return;
bp->b_flags &= ~_XBF_IN_FLIGHT;
percpu_counter_dec(&bp->b_target->bt_io_count);
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

5
fs/xfs/xfs_buf.h
View File

@ -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" -