[Qemu-devel] [PATCH v2 09/13] linux-aio: bind EventNotifier to current AioContext

[Stefan Hajnoczi <stefanha@redhat.com>]

Each block/raw-posix.c BlockDriverState has a struct qemu_laio_state
that holds the Linux AIO context.  Currently the EventNotifier is
assigned to the main loop.

In order to use block/linux-aio.c from other threads/event loops, we
need to assign the EventNotifier to the current event loop.  This patch
implements this by assigning the EventNotifier upon the first pending
request and deassigning it on the last pending request.

It assumes that we flush requests before issuing I/O from another event
loop (this is true).

This new approach could affect performance since we assign/deassign the
EventNotifier while processing I/O.  However, it is the simplest way to
support Linux AIO from multiple threads/event loops, so I think it's a
reasonable start.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/linux-aio.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/block/linux-aio.c b/block/linux-aio.c
index ee0f8d1..36633a2 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -56,6 +56,9 @@ static void qemu_laio_process_completion(struct qemu_laio_state *s,
     int ret;
 
     s->count--;
+    if (s->count == 0) {
+        qemu_aio_set_event_notifier(&s->e, NULL, NULL);
+    }
 
     ret = laiocb->ret;
     if (ret != -ECANCELED) {
@@ -176,6 +179,16 @@ BlockDriverAIOCB *laio_submit(BlockDriverState *bs, void *aio_ctx, int fd,
                         __func__, type);
         goto out_free_aiocb;
     }
+
+    /* Assign our EventNotifier to the current thread's AioContext.  This
+     * allows Linux AIO to be used from multiple threads so long as they flush
+     * before switching threads.
+     */
+    if (s->count == 0) {
+        qemu_aio_set_event_notifier(&s->e, qemu_laio_completion_cb,
+                                    qemu_laio_flush_cb);
+    }
+
     io_set_eventfd(&laiocb->iocb, event_notifier_get_fd(&s->e));
     s->count++;
 
@@ -185,6 +198,9 @@ BlockDriverAIOCB *laio_submit(BlockDriverState *bs, void *aio_ctx, int fd,
 
 out_dec_count:
     s->count--;
+    if (s->count == 0) {
+        qemu_aio_set_event_notifier(&s->e, NULL, NULL);
+    }
 out_free_aiocb:
     qemu_aio_release(laiocb);
     return NULL;
@@ -203,9 +219,6 @@ void *laio_init(void)
         goto out_close_efd;
     }
 
-    qemu_aio_set_event_notifier(&s->e, qemu_laio_completion_cb,
-                                qemu_laio_flush_cb);
-
     return s;
 
 out_close_efd:
-- 
1.8.1.4