On 1/17/24 13:14, Jens Axboe wrote:
/* Maps an I/O priority class to a deadline scheduler priority. */
@@ -600,6 +604,10 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
struct request *rq;
enum dd_prio prio;
+ if (test_bit(0, &dd->dispatch_state) ||
+ test_and_set_bit(0, &dd->dispatch_state))
+ return NULL;
+
spin_lock(&dd->lock);
rq = dd_dispatch_prio_aged_requests(dd, now);
if (rq)
@@ -616,6 +624,7 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
}
unlock:
+ clear_bit(0, &dd->dispatch_state);
spin_unlock(&dd->lock);
Can the above code be simplified by using spin_trylock() instead of
test_bit() and test_and_set_bit()?
Please note that whether or not spin_trylock() is used, there is a
race condition in this approach: if dd_dispatch_request() is called
just before another CPU calls spin_unlock() from inside
dd_dispatch_request() then some requests won't be dispatched until
the next time dd_dispatch_request() is called.
Thanks,
Bart.
