[Qemu-devel] [RFC PATCH v2 6/6] linux-aio: Queue requests instead of returning -EAGAIN

[Kevin Wolf]

If the queue array for io_submit() is already full, but a new request
arrives, we cannot add it to that queue anymore. We can, however, use a
CoQueue, which is implemented as a list and can therefore queue as many
requests as we want.

Signed-off-by: Kevin Wolf <address@hidden>
---
 block/linux-aio.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/block/linux-aio.c b/block/linux-aio.c
index 373ec4b..8e6328b 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -44,6 +44,7 @@ typedef struct {
     int plugged;
     unsigned int size;
     unsigned int idx;
+    CoQueue waiting;
 } LaioQueue;
 
 struct qemu_laio_state {
@@ -160,6 +161,8 @@ static void ioq_init(LaioQueue *io_q)
     io_q->size = MAX_QUEUED_IO;
     io_q->idx = 0;
     io_q->plugged = 0;
+
+    qemu_co_queue_init(&io_q->waiting);
 }
 
 static int ioq_submit(struct qemu_laio_state *s)
@@ -201,15 +204,29 @@ static int ioq_submit(struct qemu_laio_state *s)
                 s->io_q.idx * sizeof(s->io_q.iocbs[0]));
     }
 
+    /* Now there should be room for some more requests */
+    if (!qemu_co_queue_empty(&s->io_q.waiting)) {
+        if (qemu_in_coroutine()) {
+            qemu_co_queue_next(&s->io_q.waiting);
+        } else {
+            qemu_co_enter_next(&s->io_q.waiting);
+
+        }
+    }
+
     return ret;
 }
 
 static int ioq_enqueue(struct qemu_laio_state *s, struct iocb *iocb)
 {
     unsigned int idx = s->io_q.idx;
+    bool was_waiting = false;
+    int ret = 0;
 
-    if (unlikely(idx == s->io_q.size)) {
-        return -EAGAIN;
+    while (unlikely(idx == s->io_q.size)) {
+        /* Wait for iocb slots to become free */
+        qemu_co_queue_wait(&s->io_q.waiting);
+        was_waiting = true;
     }
 
     s->io_q.iocbs[idx++] = iocb;
@@ -217,10 +234,14 @@ static int ioq_enqueue(struct qemu_laio_state *s, struct 
iocb *iocb)
 
     /* submit immediately if queue is not plugged or full */
     if (!s->io_q.plugged || idx == s->io_q.size) {
-        return ioq_submit(s);
-    } else {
-        return 0;
+        ret = ioq_submit(s);
+    } else if (was_waiting) {
+        /* We were just woken up and there's stil room in the queue. Wake up
+         * someone else, too. */
+        qemu_co_queue_next(&s->io_q.waiting);
     }
+
+    return ret;
 }
 
 void laio_io_plug(BlockDriverState *bs, void *aio_ctx)
-- 
1.8.3.1