[Qemu-devel] [RFC 4/5] block: Add AIO followers

[Max Reitz]

If a long-running operation on a BDS wants to always remain in the same
AIO context, it somehow needs to keep track of the BDS changing its
context. This adds a function for registering callbacks on a BDS which
are called whenever the BDS is attached or detached from an AIO context.

Signed-off-by: Max Reitz <address@hidden>
---
 block.c                   | 55 +++++++++++++++++++++++++++++++++++++++++++++++
 include/block/block_int.h | 40 ++++++++++++++++++++++++++++++++++
 2 files changed, 95 insertions(+)

diff --git a/block.c b/block.c
index fe85d78..9c8dd5b 100644
--- a/block.c
+++ b/block.c
@@ -1775,6 +1775,8 @@ void bdrv_reopen_abort(BDRVReopenState *reopen_state)
 
 void bdrv_close(BlockDriverState *bs)
 {
+    BdrvAioFollower *baf, *baf_next;
+
     if (bs->job) {
         block_job_cancel_sync(bs->job);
     }
@@ -1816,6 +1818,11 @@ void bdrv_close(BlockDriverState *bs)
     if (bs->io_limits_enabled) {
         bdrv_io_limits_disable(bs);
     }
+
+    QLIST_FOREACH_SAFE(baf, &bs->aio_followers, list, baf_next) {
+        g_free(baf);
+    }
+    QLIST_INIT(&bs->aio_followers);
 }
 
 void bdrv_close_all(void)
@@ -5580,10 +5587,16 @@ AioContext *bdrv_get_aio_context(BlockDriverState *bs)
 
 void bdrv_detach_aio_context(BlockDriverState *bs)
 {
+    BdrvAioFollower *baf;
+
     if (!bs->drv) {
         return;
     }
 
+    QLIST_FOREACH(baf, &bs->aio_followers, list) {
+        baf->aio_context_detached(baf->opaque);
+    }
+
     if (bs->drv->bdrv_detach_aio_context) {
         bs->drv->bdrv_detach_aio_context(bs);
     }
@@ -5600,6 +5613,8 @@ void bdrv_detach_aio_context(BlockDriverState *bs)
 void bdrv_attach_aio_context(BlockDriverState *bs,
                              AioContext *new_context)
 {
+    BdrvAioFollower *baf;
+
     if (!bs->drv) {
         return;
     }
@@ -5615,6 +5630,10 @@ void bdrv_attach_aio_context(BlockDriverState *bs,
     if (bs->drv->bdrv_attach_aio_context) {
         bs->drv->bdrv_attach_aio_context(bs, new_context);
     }
+
+    QLIST_FOREACH(baf, &bs->aio_followers, list) {
+        baf->aio_context_attached(new_context, baf->opaque);
+    }
 }
 
 void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context)
@@ -5631,6 +5650,42 @@ void bdrv_set_aio_context(BlockDriverState *bs, 
AioContext *new_context)
     aio_context_release(new_context);
 }
 
+void bdrv_follow_aio(BlockDriverState *bs,
+                     void (*aio_context_attached)(AioContext *new_context,
+                                                  void *opaque),
+                     void (*aio_context_detached)(void *opaque), void *opaque)
+{
+    BdrvAioFollower *baf = g_new(BdrvAioFollower, 1);
+    *baf = (BdrvAioFollower){
+        .aio_context_attached = aio_context_attached,
+        .aio_context_detached = aio_context_detached,
+        .opaque               = opaque
+    };
+
+    QLIST_INSERT_HEAD(&bs->aio_followers, baf, list);
+}
+
+void bdrv_unfollow_aio(BlockDriverState *bs,
+                       void (*aio_context_attached)(AioContext *, void *),
+                       void (*aio_context_detached)(void *), void *opaque)
+{
+    BdrvAioFollower *baf, *baf_next;
+
+    QLIST_FOREACH_SAFE(baf, &bs->aio_followers, list, baf_next) {
+        if (baf->aio_context_attached == aio_context_attached &&
+            baf->aio_context_detached == aio_context_detached &&
+            baf->opaque               == opaque)
+        {
+            QLIST_REMOVE(baf, list);
+            g_free(baf);
+
+            return;
+        }
+    }
+
+    abort();
+}
+
 void bdrv_add_before_write_notifier(BlockDriverState *bs,
                                     NotifierWithReturn *notifier)
 {
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 7d156e3..aacc22f 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -283,6 +283,15 @@ typedef struct BlockLimits {
     size_t opt_mem_alignment;
 } BlockLimits;
 
+typedef struct BdrvAioFollower {
+    void (*aio_context_attached)(AioContext *new_context, void *opaque);
+    void (*aio_context_detached)(void *opaque);
+
+    void *opaque;
+
+    QLIST_ENTRY(BdrvAioFollower) list;
+} BdrvAioFollower;
+
 /*
  * Note: the function bdrv_append() copies and swaps contents of
  * BlockDriverStates, so if you add new fields to this struct, please
@@ -309,6 +318,10 @@ struct BlockDriverState {
     void *dev_opaque;
 
     AioContext *aio_context; /* event loop used for fd handlers, timers, etc */
+    /* long-running tasks intended to always use the same AioContext as this
+     * BDS may register themselves in this list to be notified of changes
+     * regarding this BDS's context */
+    QLIST_HEAD(, BdrvAioFollower) aio_followers;
 
     char filename[1024];
     char backing_file[1024]; /* if non zero, the image is a diff of
@@ -425,6 +438,33 @@ void bdrv_detach_aio_context(BlockDriverState *bs);
 void bdrv_attach_aio_context(BlockDriverState *bs,
                              AioContext *new_context);
 
+/**
+ * bdrv_follow_aio:
+ *
+ * If a long-running job intends to be always run in the same AioContext as a
+ * certain BDS, it may use this function to be notified of changes regarding 
the
+ * association of the BDS to an AioContext.
+ *
+ * aio_context_attached() is called after the target BDS has been attached to a
+ * new AioContext; aio_context_detached() is called before the target BDS is
+ * being detached from its old AioContext.
+ */
+void bdrv_follow_aio(BlockDriverState *bs,
+                     void (*aio_context_attached)(AioContext *new_context,
+                                                  void *opaque),
+                     void (*aio_context_detached)(void *opaque), void *opaque);
+
+/**
+ * bdrv_unfollow_aio:
+ *
+ * Unsubscribe of change notifications regarding the BDS's AioContext. The
+ * parameters given here have to be the same as those given to
+ * bdrv_follow_aio().
+ */
+void bdrv_unfollow_aio(BlockDriverState *bs,
+                       void (*aio_context_attached)(AioContext *, void *),
+                       void (*aio_context_detached)(void *), void *opaque);
+
 #ifdef _WIN32
 int is_windows_drive(const char *filename);
 #endif
-- 
1.9.3