linux-aio: properly bubble up errors from initialization

laio_init() can fail for a couple of reasons, which will lead to a NULL
pointer dereference in laio_attach_aio_context().

To solve this, add a aio_setup_linux_aio() function which is called
early in raw_open_common. If this fails, propagate the error up. The
signature of aio_get_linux_aio() was not modified, because it seems
preferable to return the actual errno from the possible failing
initialization calls.

Additionally, when the AioContext changes, we need to associate a
LinuxAioState with the new AioContext. Use the bdrv_attach_aio_context
callback and call the new aio_setup_linux_aio(), which will allocate a
new AioContext if needed, and return errors on failures. If it fails for
any reason, fallback to threaded AIO with an error message, as the
device is already in-use by the guest.

Add an assert that aio_get_linux_aio() cannot return NULL.

Signed-off-by: Nishanth Aravamudan <naravamudan@digitalocean.com>
Message-id: 20180622193700.6523-1-naravamudan@digitalocean.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 11 insertions, 3 deletions

diff --git a/util/async.c b/util/async.c
index 03f62787f2..05979f8014 100644
--- a/util/async.c
+++ b/util/async.c
@@ -323,14 +323,22 @@ ThreadPool *aio_get_thread_pool(AioContext *ctx)
 }
 
 #ifdef CONFIG_LINUX_AIO
-LinuxAioState *aio_get_linux_aio(AioContext *ctx)
+LinuxAioState *aio_setup_linux_aio(AioContext *ctx, Error **errp)
 {
     if (!ctx->linux_aio) {
-        ctx->linux_aio = laio_init();
-        laio_attach_aio_context(ctx->linux_aio, ctx);
+        ctx->linux_aio = laio_init(errp);
+        if (ctx->linux_aio) {
+            laio_attach_aio_context(ctx->linux_aio, ctx);
+        }
     }
     return ctx->linux_aio;
 }
+
+LinuxAioState *aio_get_linux_aio(AioContext *ctx)
+{
+    assert(ctx->linux_aio);
+    return ctx->linux_aio;
+}
 #endif
 
 void aio_notify(AioContext *ctx)