David Howells <dhowells@xxxxxxxxxx> wrote:
> I guess I need to stick a comment in slow_work_enqueue() to detail this,
> though the comments in slow_work_execute() do talk about it.
How about this:
diff --git a/kernel/slow-work.c b/kernel/slow-work.c
index f638f36..adf9b78 100644
--- a/kernel/slow-work.c
+++ b/kernel/slow-work.c
@@ -195,12 +195,32 @@ int slow_work_enqueue(struct slow_work *work)
BUG_ON(!work->ops);
BUG_ON(!work->ops->get_ref);
+ /* when honouring an enqueue request, we only promise that we will run
+ * the work function in the future; we do not promise to run it once
+ * per enqueue request
+ *
+ * we use the PENDING bit to merge together repeat requests without
+ * having to disable IRQs and take the spinlock, whilst still
+ * maintaining our promise
+ */
if (!test_and_set_bit_lock(SLOW_WORK_PENDING, &work->flags)) {
spin_lock_irqsave(&slow_work_queue_lock, flags);
+ /* we promise that we will not attempt to execute the work
+ * function in more than one thread simultaneously
+ *
+ * this, however, leaves us with a problem if we're asked to
+ * enqueue the work whilst someone is executing the work
+ * function as simply queueing the work immediately means that
+ * another thread may try executing it whilst it is already
+ * under execution
+ *
+ * to deal with this, we set the ENQ_DEFERRED bit instead of
+ * enqueueing, and the thread currently executing the work
+ * function will enqueue the work item when the work function
+ * returns and it has cleared the EXECUTING bit
+ */
if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) {
- /* can't queue lest we cause multiple threads to try
- * executing this item, so defer for later */
set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags);
} else {
if (work->ops->get_ref(work) < 0)
David
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