aio: add a ThreadPool instance to AioContext

This patch adds a ThreadPool to AioContext.  It's possible that some
AioContext instances will never use the ThreadPool, so defer creation
until aio_get_thread_pool().

The reason why AioContext should have the ThreadPool is because the
ThreadPool is bound to a AioContext instance where the work item's
callback function is invoked.  It doesn't make sense to keep the
ThreadPool pointer anywhere other than AioContext.  For example,
block/raw-posix.c can get its AioContext's ThreadPool and submit work.

Special note about headers: I used struct ThreadPool in aio.h because
there is a circular dependency if aio.h includes thread-pool.h.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

async.c

@@ -24,6 +24,7 @@
 
 #include "qemu-common.h"
 #include "block/aio.h"
+#include "block/thread-pool.h"
 #include "qemu/main-loop.h"
 
 /***********************************************************/
@@ -172,6 +173,7 @@ aio_ctx_finalize(GSource     *source)
 {
     AioContext *ctx = (AioContext *) source;
 
+    thread_pool_free(ctx->thread_pool);
     aio_set_event_notifier(ctx, &ctx->notifier, NULL, NULL);
     event_notifier_cleanup(&ctx->notifier);
     g_array_free(ctx->pollfds, TRUE);
@@ -190,6 +192,14 @@ GSource *aio_get_g_source(AioContext *ctx)
     return &ctx->source;
 }
 
+ThreadPool *aio_get_thread_pool(AioContext *ctx)
+{
+    if (!ctx->thread_pool) {
+        ctx->thread_pool = thread_pool_new(ctx);
+    }
+    return ctx->thread_pool;
+}
+
 void aio_notify(AioContext *ctx)
 {
     event_notifier_set(&ctx->notifier);
@@ -200,6 +210,7 @@ AioContext *aio_context_new(void)
     AioContext *ctx;
     ctx = (AioContext *) g_source_new(&aio_source_funcs, sizeof(AioContext));
     ctx->pollfds = g_array_new(FALSE, FALSE, sizeof(GPollFD));
+    ctx->thread_pool = NULL;
     event_notifier_init(&ctx->notifier, false);
     aio_set_event_notifier(ctx, &ctx->notifier, 
                            (EventNotifierHandler *)

include/block/aio.h

@@ -66,6 +66,9 @@ typedef struct AioContext {
 
     /* GPollFDs for aio_poll() */
     GArray *pollfds;
+
+    /* Thread pool for performing work and receiving completion callbacks */
+    struct ThreadPool *thread_pool;
 } AioContext;
 
 /* Returns 1 if there are still outstanding AIO requests; 0 otherwise */
@@ -223,6 +226,9 @@ void aio_set_event_notifier(AioContext *ctx,
  */
 GSource *aio_get_g_source(AioContext *ctx);
 
+/* Return the ThreadPool bound to this AioContext */
+struct ThreadPool *aio_get_thread_pool(AioContext *ctx);
+
 /* Functions to operate on the main QEMU AioContext.  */
 
 bool qemu_aio_wait(void);