The tail of a queue is supposed to be pointing to the next available
slot in a queue.  In this implementation the tail is incremented before
it is used and as such points to the last used element, something that
has the immense advantage of centralizing tail management at a single
location and eliminating a lot of redundant code.

But this needs to be taken into consideration on the dequeueing side
where the head also needs to be incremented before it is used, or the
first available element of the queue will be skipped.
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Tested-by: Leo Yan <leo.yan@linaro.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Robert Walker <robert.walker@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1527289854-10755-1-git-send-email-mathieu.poirier@linaro.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>