On Fri, Jun 15, 2018 at 02:05:36PM -0400, Brian Foster wrote:
> If a delwri queue occurs of a buffer that sits on a delwri queue
> wait list, the queue sets _XBF_DELWRI_Q without changing the state
> of ->b_list. This occurs, for example, if another thread beats the
> current delwri waiter thread to the buffer lock after I/O
> completion. Once the waiter acquires the lock, it removes the buffer
> from the wait list and leaves a buffer with _XBF_DELWRI_Q set but
> not populated on a list. This results in a lost buffer submission
> and in turn can result in assert failures due to _XBF_DELWRI_Q being
> set on buffer reclaim or filesystem lockups if the buffer happens to
> cover an item in the AIL.
>
> This problem has been reproduced by repeated iterations of xfs/305
> on high CPU count (28xcpu) systems with limited memory (~1GB). Dirty
> dquot reclaim races with an xfsaild push of a separate dquot backed
> by the same buffer such that the buffer sits on the reclaim wait
> list at the time xfsaild attempts to queue it. Since the latter
> dquot has been flush locked but the underlying buffer not submitted
> for I/O, the dquot pins the AIL and causes the filesystem to
> livelock.
>
> This race is essentially made possible by the buffer lock cycle
> involved with waiting on a synchronous delwri queue submission.
> Close the race by using synchronous buffer I/O for respective delwri
> queue submission. This means the buffer remains locked across the
> I/O and so is inaccessible from other contexts while in the
> intermediate wait list state. The sync buffer I/O wait mechanism is
> factored into a helper such that sync delwri buffer submission and
> serialization are batched operations.
>
> Designed-by: Dave Chinner <dchinner@xxxxxxxxxx>
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> ---
> fs/xfs/xfs_buf.c | 78 +++++++++++++++++++++++++-----------------------
> 1 file changed, 41 insertions(+), 37 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 7b0f7c79cd62..b5c0f6949b30 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -1531,6 +1531,20 @@ xfs_buf_submit(
> xfs_buf_rele(bp);
> }
>
> +/*
> + * Wait for I/O completion of a sync buffer and return the I/O error code.
> + */
> +static int
> +xfs_buf_iowait(
> + struct xfs_buf *bp)
> +{
> + trace_xfs_buf_iowait(bp, _RET_IP_);
> + wait_for_completion(&bp->b_iowait);
> + trace_xfs_buf_iowait_done(bp, _RET_IP_);
> +
> + return bp->b_error;
> +}
> +
> /*
> * Synchronous buffer IO submission path, read or write.
> */
> @@ -1553,12 +1567,7 @@ xfs_buf_submit_wait(
> error = __xfs_buf_submit(bp);
> if (error)
> goto out;
> -
> - /* wait for completion before gathering the error from the buffer */
> - trace_xfs_buf_iowait(bp, _RET_IP_);
> - wait_for_completion(&bp->b_iowait);
> - trace_xfs_buf_iowait_done(bp, _RET_IP_);
> - error = bp->b_error;
> + error = xfs_buf_iowait(bp);
>
> out:
> /*
> @@ -1961,16 +1970,11 @@ xfs_buf_cmp(
> }
>
> /*
> - * submit buffers for write.
> - *
> - * When we have a large buffer list, we do not want to hold all the buffers
> - * locked while we block on the request queue waiting for IO dispatch. To avoid
> - * this problem, we lock and submit buffers in groups of 50, thereby minimising
> - * the lock hold times for lists which may contain thousands of objects.
> - *
> - * To do this, we sort the buffer list before we walk the list to lock and
> - * submit buffers, and we plug and unplug around each group of buffers we
> - * submit.
> + * Submit buffers for write. If wait_list is specified, the buffers are
> + * submitted using sync I/O and placed on the wait list such that the caller can
> + * iowait each buffer. Otherwise async I/O is used and the buffers are released
> + * at I/O completion time. In either case, buffers remain locked until I/O
> + * completes and the buffer is released from the queue.
> */
> static int
> xfs_buf_delwri_submit_buffers(
> @@ -2012,21 +2016,22 @@ xfs_buf_delwri_submit_buffers(
> trace_xfs_buf_delwri_split(bp, _RET_IP_);
>
> /*
> - * We do all IO submission async. This means if we need
> - * to wait for IO completion we need to take an extra
> - * reference so the buffer is still valid on the other
> - * side. We need to move the buffer onto the io_list
> - * at this point so the caller can still access it.
> + * If we have a wait list, each buffer (and associated delwri
> + * queue reference) transfers to it and is submitted
> + * synchronously. Otherwise, drop the buffer from the delwri
> + * queue and submit async.
> */
> bp->b_flags &= ~(_XBF_DELWRI_Q | XBF_WRITE_FAIL);
> - bp->b_flags |= XBF_WRITE | XBF_ASYNC;
> + bp->b_flags |= XBF_WRITE;
> if (wait_list) {
> - xfs_buf_hold(bp);
> + bp->b_flags &= ~XBF_ASYNC;
> list_move_tail(&bp->b_list, wait_list);
> - } else
> + __xfs_buf_submit(bp);
> + } else {
> + bp->b_flags |= XBF_ASYNC;
> list_del_init(&bp->b_list);
> -
> - xfs_buf_submit(bp);
> + xfs_buf_submit(bp);
> + }
> }
> blk_finish_plug(&plug);
>
> @@ -2073,8 +2078,11 @@ xfs_buf_delwri_submit(
>
> list_del_init(&bp->b_list);
>
> - /* locking the buffer will wait for async IO completion. */
> - xfs_buf_lock(bp);
> + /*
> + * Wait on the locked buffer, check for errors and unlock and
> + * release the delwri queue reference.
> + */
> + xfs_buf_iowait(bp);
> error2 = bp->b_error;
error2 = xfs_buf_iowait()?
> xfs_buf_relse(bp);
> if (!error)
> @@ -2121,22 +2129,18 @@ xfs_buf_delwri_pushbuf(
>
> /*
> * Delwri submission clears the DELWRI_Q buffer flag and returns with
> - * the buffer on the wait list with an associated reference. Rather than
> + * the buffer on the wait list with the original reference. Rather than
> * bounce the buffer from a local wait list back to the original list
> * after I/O completion, reuse the original list as the wait list.
> */
> xfs_buf_delwri_submit_buffers(&submit_list, buffer_list);
>
> /*
> - * The buffer is now under I/O and wait listed as during typical delwri
> - * submission. Lock the buffer to wait for I/O completion. Rather than
> - * remove the buffer from the wait list and release the reference, we
> - * want to return with the buffer queued to the original list. The
> - * buffer already sits on the original list with a wait list reference,
> - * however. If we let the queue inherit that wait list reference, all we
> - * need to do is reset the DELWRI_Q flag.
> + * The buffer is now locked, under I/O and wait listed on the original
> + * delwri queue. Wait for I/O completion, restore the DELWRI_Q flag and
> + * return with the buffer unlocked and on the original queue.
> */
> - xfs_buf_lock(bp);
> + xfs_buf_iowait(bp);
> error = bp->b_error;
error = xfs_buf_iowait()?
Or is there a specific reason you collected the error separately?
Looks reasonable and I could follow along this time. :)
--D
> bp->b_flags |= _XBF_DELWRI_Q;
> xfs_buf_unlock(bp);
> --
> 2.17.1
>
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