block: add polled wakeup task helper

If we're polling for IO on a device that doesn't use interrupts, then
IO completion loop (and wake of task) is done by submitting task itself.
If that is the case, then we don't need to enter the wake_up_process()
function, we can simply mark ourselves as TASK_RUNNING.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -1772,4 +1772,17 @@ static inline int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
 
 #endif /* CONFIG_BLOCK */
 
+static inline void blk_wake_io_task(struct task_struct *waiter)
+{
+	/*
+	 * If we're polling, the task itself is doing the completions. For
+	 * that case, we don't need to signal a wakeup, it's enough to just
+	 * mark us as RUNNING.
+	 */
+	if (waiter == current)
+		__set_current_state(TASK_RUNNING);
+	else
+		wake_up_process(waiter);
+}
+
 #endif