[PATCH V14 03/15] iothread: add holder-aware AioContext accessors

[Zhang Chen <zhangckid@gmail.com>]

Introduce iothread_ref_and_get_aio_context() with a holder argument
and its counterpart iothread_unref_and_put_aio_context().

Previously, users of an IOThread AioContext did not explicitly record
their identity, making it difficult to debug which devices or
subsystems were pinning an IOThread. These new helpers provide the
infrastructure for callers to register their identity. Subsequent
patches convert existing users to the new helpers.

Registering a holder takes an IOThread object reference so that the
IOThread and its AioContext stay alive until the matching put
operation. Document the ownership and BQL requirements.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Zhang Chen <zhangckid@gmail.com>
---
 include/system/iothread.h | 22 ++++++++++++++++++++++
 iothread.c                | 24 +++++++++++++++++++++++-
 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/include/system/iothread.h b/include/system/iothread.h
index b8aeb32b0e..f6c5f95dd9 100644
--- a/include/system/iothread.h
+++ b/include/system/iothread.h
@@ -67,6 +67,28 @@ DECLARE_INSTANCE_CHECKER(IOThread, IOTHREAD,
 char *iothread_get_id(IOThread *iothread);
 IOThread *iothread_by_id(const char *id);
 AioContext *iothread_get_aio_context(IOThread *iothread);
+
+/*
+ * Register @holder and return @iothread's AioContext.  The holder is copied,
+ * and a reference is taken on @iothread so that both the IOThread and its
+ * AioContext remain alive.
+ *
+ * The caller must eventually call iothread_unref_and_put_aio_context() with an
+ * equivalent holder.  This function is not thread-safe and must be called
+ * under the Big QEMU Lock (BQL).
+ */
+AioContext *iothread_ref_and_get_aio_context(IOThread *iothread,
+                                             const IOThreadHolder *holder);
+
+/*
+ * Unregister @holder and release the corresponding reference on @iothread.
+ * Calling this function without a matching
+ * iothread_ref_and_get_aio_context() call is a programming error.
+ *
+ * This function is not thread-safe and must be called under the BQL.
+ */
+void iothread_unref_and_put_aio_context(IOThread *iothread,
+                                        const IOThreadHolder *holder);
 GMainContext *iothread_get_g_main_context(IOThread *iothread);
 
 /*
diff --git a/iothread.c b/iothread.c
index 66dfde626e..58dace351c 100644
--- a/iothread.c
+++ b/iothread.c
@@ -33,6 +33,11 @@ void iothread_ref(IOThread *iothread, const IOThreadHolder *holder)
     assert(holder);
 
     QAPI_LIST_PREPEND(iothread->holders, QAPI_CLONE(IOThreadHolder, holder));
+    /*
+     * This guarantees that the IOThread and its AioContext remain alive
+     * as long as there is a holder.
+     */
+    object_ref(OBJECT(iothread));
 }
 
 static int iothread_holder_compare(const IOThreadHolder *holder_a,
@@ -78,6 +83,7 @@ void iothread_unref(IOThread *iothread, const IOThreadHolder *holder)
             *prev = curr->next;
             curr->next = NULL;
             qapi_free_IOThreadHolderList(curr);
+            object_unref(OBJECT(iothread));
             return;
         }
         prev = &curr->next;
@@ -199,7 +205,7 @@ static void iothread_init_gcontext(IOThread *iothread, const char *thread_name)
     g_autofree char *name = g_strdup_printf("%s aio-context", thread_name);
 
     iothread->worker_context = g_main_context_new();
-    source = aio_get_g_source(iothread_get_aio_context(iothread));
+    source = aio_get_g_source(iothread->ctx);
     g_source_set_name(source, name);
     g_source_attach(source, iothread->worker_context);
     g_source_unref(source);
@@ -421,6 +427,22 @@ AioContext *iothread_get_aio_context(IOThread *iothread)
     return iothread->ctx;
 }
 
+AioContext *iothread_ref_and_get_aio_context(IOThread *iothread,
+                                             const IOThreadHolder *holder)
+{
+    /* Add IOThreadHolder to the list */
+    iothread_ref(iothread, holder);
+
+    return iothread->ctx;
+}
+
+void iothread_unref_and_put_aio_context(IOThread *iothread,
+                                        const IOThreadHolder *holder)
+{
+    /* Delete IOThreadHolder from the list */
+    iothread_unref(iothread, holder);
+}
+
 static int query_one_iothread(Object *object, void *opaque)
 {
     IOThreadInfoList ***tail = opaque;
-- 
2.55.0