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    Subject[PATCH 4.4 46/73] xfs: prevent dropping ioend completions during buftarg wait
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Brian Foster <bfoster@redhat.com>

    commit 800b2694f890cc35a1bda63501fc71c94389d517 upstream.

    xfs_wait_buftarg() waits for all pending I/O, drains the ioend
    completion workqueue and walks the LRU until all buffers in the cache
    have been released. This is traditionally an unmount operation` but the
    mechanism is also reused during filesystem freeze.

    xfs_wait_buftarg() invokes drain_workqueue() as part of the quiesce,
    which is intended more for a shutdown sequence in that it indicates to
    the queue that new operations are not expected once the drain has begun.
    New work jobs after this point result in a WARN_ON_ONCE() and are
    otherwise dropped.

    With filesystem freeze, however, read operations are allowed and can
    proceed during or after the workqueue drain. If such a read occurs
    during the drain sequence, the workqueue infrastructure complains about
    the queued ioend completion work item and drops it on the floor. As a
    result, the buffer remains on the LRU and the freeze never completes.

    Despite the fact that the overall buffer cache cleanup is not necessary
    during freeze, fix up this operation such that it is safe to invoke
    during non-unmount quiesce operations. Replace the drain_workqueue()
    call with flush_workqueue(), which runs a similar serialization on
    pending workqueue jobs without causing new jobs to be dropped. This is
    safe for unmount as unmount independently locks out new operations by
    the time xfs_wait_buftarg() is invoked.

    cc: <stable@vger.kernel.org>
    Signed-off-by: Brian Foster <bfoster@redhat.com>
    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Signed-off-by: Dave Chinner <david@fromorbit.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/xfs/xfs_buf.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/fs/xfs/xfs_buf.c
    +++ b/fs/xfs/xfs_buf.c
    @@ -1535,7 +1535,7 @@ xfs_wait_buftarg(
    * ensure here that all reference counts have been dropped before we
    * start walking the LRU list.
    */
    - drain_workqueue(btp->bt_mount->m_buf_workqueue);
    + flush_workqueue(btp->bt_mount->m_buf_workqueue);

    /* loop until there is nothing left on the lru list. */
    while (list_lru_count(&btp->bt_lru)) {

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