[Qemu-devel] [PATCH v4 1/8] block: add bdrv_add_before_write_notifier()

[Stefan Hajnoczi]

The bdrv_add_before_write_notifier() function installs a callback that
is invoked before a write request is processed.  This will be used to
implement copy-on-write point-in-time snapshots where we need to copy
out old data before overwriting it.

Note that BdrvTrackedRequest is moved to block_int.h since it is passed
to .notify() functions.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 block.c                   | 18 ++++++++----------
 include/block/block_int.h | 22 +++++++++++++++++++++-
 2 files changed, 29 insertions(+), 11 deletions(-)

diff --git a/block.c b/block.c
index 3f87489..591e312 100644
--- a/block.c
+++ b/block.c
@@ -308,6 +308,7 @@ BlockDriverState *bdrv_new(const char *device_name)
     }
     bdrv_iostatus_disable(bs);
     notifier_list_init(&bs->close_notifiers);
+    notifier_list_init(&bs->before_write_notifiers);
 
     return bs;
 }
@@ -1839,16 +1840,6 @@ int bdrv_commit_all(void)
     return 0;
 }
 
-struct BdrvTrackedRequest {
-    BlockDriverState *bs;
-    int64_t sector_num;
-    int nb_sectors;
-    bool is_write;
-    QLIST_ENTRY(BdrvTrackedRequest) list;
-    Coroutine *co; /* owner, used for deadlock detection */
-    CoQueue wait_queue; /* coroutines blocked on this request */
-};
-
 /**
  * Remove an active request from the tracked requests list
  *
@@ -2619,6 +2610,8 @@ static int coroutine_fn 
bdrv_co_do_writev(BlockDriverState *bs,
 
     tracked_request_begin(&req, bs, sector_num, nb_sectors, true);
 
+    notifier_list_notify(&bs->before_write_notifiers, &req);
+
     if (flags & BDRV_REQ_ZERO_WRITE) {
         ret = bdrv_co_do_write_zeroes(bs, sector_num, nb_sectors);
     } else {
@@ -4883,3 +4876,8 @@ AioContext *bdrv_get_aio_context(BlockDriverState *bs)
     /* Currently BlockDriverState always uses the main loop AioContext */
     return qemu_get_aio_context();
 }
+
+void bdrv_add_before_write_notifier(BlockDriverState *bs, Notifier *notifier)
+{
+    notifier_list_add(&bs->before_write_notifiers, notifier);
+}
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 6078dd3..a498fb0 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -58,7 +58,16 @@
 #define BLOCK_OPT_LAZY_REFCOUNTS    "lazy_refcounts"
 #define BLOCK_OPT_ADAPTER_TYPE      "adapter_type"
 
-typedef struct BdrvTrackedRequest BdrvTrackedRequest;
+typedef struct BdrvTrackedRequest {
+    BlockDriverState *bs;
+    int64_t sector_num;
+    int nb_sectors;
+    bool is_write;
+    QLIST_ENTRY(BdrvTrackedRequest) list;
+    Coroutine *co; /* owner, used for deadlock detection */
+    CoQueue wait_queue; /* coroutines blocked on this request */
+} BdrvTrackedRequest;
+
 
 typedef struct BlockIOLimit {
     int64_t bps[3];
@@ -247,6 +256,9 @@ struct BlockDriverState {
 
     NotifierList close_notifiers;
 
+    /* Callback before write request is processed */
+    NotifierList before_write_notifiers;
+
     /* number of in-flight copy-on-read requests */
     unsigned int copy_on_read_in_flight;
 
@@ -298,6 +310,14 @@ void bdrv_set_io_limits(BlockDriverState *bs,
                         BlockIOLimit *io_limits);
 
 /**
+ * bdrv_add_before_write_notifier:
+ *
+ * Register a callback that is invoked before write requests are processed but
+ * after any throttling or waiting for overlapping requests.
+ */
+void bdrv_add_before_write_notifier(BlockDriverState *bs, Notifier *notifier);
+
+/**
  * bdrv_get_aio_context:
  *
  * Returns: the currently bound #AioContext
-- 
1.8.1.4