xfs: fix missed wakeup on l_flush_wait

xfs: fix missed wakeup on l_flush_wait

The code in xlog_wait uses the spinlock to make adding the task to
the wait queue, and setting the task state to UNINTERRUPTIBLE atomic
with respect to the waker.

Doing the wakeup after releasing the spinlock opens up the following
race condition:

Task 1					task 2
add task to wait queue
					wake up task
set task state to UNINTERRUPTIBLE

This issue was found through code inspection as a result of kworkers
being observed stuck in UNINTERRUPTIBLE state with an empty
wait queue. It is rare and largely unreproducable.

Simply moving the spin_unlock to after the wake_up_all results
in the waker not being able to see a task on the waitqueue before
it has set its state to UNINTERRUPTIBLE.

This bug dates back to the conversion of this code to generic
waitqueue infrastructure from a counting semaphore back in 2008
which didn't place the wakeups consistently w.r.t. to the relevant
spin locks.

[dchinner: Also fix a similar issue in the shutdown path on
xc_commit_wait. Update commit log with more details of the issue.]

Fixes: d748c62367eb ("[XFS] Convert l_flushsema to a sv_t")
Reported-by: Chris Mason <clm@fb.com>
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>