block-coroutine-wrapper.py: introduce annotations that take the graph rdlock

Add co_wrapper_bdrv_rdlock and co_wrapper_mixed_bdrv_rdlock option to
the block-coroutine-wrapper.py script.

This "_bdrv_rdlock" option takes and releases the graph rdlock when a
coroutine function is created.

This means that when used together with "_mixed", the function marked
with co_wrapper_mixed_bdrv_rdlock will support both coroutine and
non-coroutine case, and in the latter case it will create a coroutine
that takes and releases the rdlock. When called from a coroutine, the
caller must already hold the graph lock.

Example:
void co_wrapper_mixed_bdrv_rdlock bdrv_f1();

Becomes

static void bdrv_co_enter_f1()
{
    bdrv_graph_co_rdlock();
    bdrv_co_function();
    bdrv_graph_co_rdunlock();
}

void bdrv_f1()
{
    if (qemu_in_coroutine) {
        assume_graph_lock();
        bdrv_co_function();
    } else {
        qemu_co_enter(bdrv_co_enter_f1);
        ...
    }
}

When used alone, the function will not work in coroutine context, and
when called in non-coroutine context it will create a new coroutine that
takes care of taking and releasing the rdlock automatically.

Example:
void co_wrapper_bdrv_rdlock bdrv_f1();

Becomes

static void bdrv_co_enter_f1()
{
    bdrv_graph_co_rdlock();
    bdrv_co_function();
    bdrv_graph_co_rdunlock();
}

void bdrv_f1()
{
    assert(!qemu_in_coroutine());
    qemu_co_enter(bdrv_co_enter_f1);
    ...
}

About their usage:
- co_wrapper does not take the rdlock, so it can be used also outside
  the block layer.
- co_wrapper_mixed will be used by many blk_* functions, since the
  coroutine function needs to call blk_wait_while_drained() and
  the rdlock *must* be taken afterwards, otherwise it's a deadlock.
  In the future this annotation will go away, and blk_* will use
  co_wrapper directly.
- co_wrapper_bdrv_rdlock will be used by BlockDriver callbacks, ideally
  by all of them in the future.
- co_wrapper_mixed_bdrv_rdlock will be used by the remaining functions
  that are still called by coroutine and non-coroutine context. In the
  future this annotation will go away, as we will split such mixed
  functions.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20221207131838.239125-17-kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/block/block-common.h

@@ -40,14 +40,21 @@
  *
  * Usage: read docs/devel/block-coroutine-wrapper.rst
  *
- * There are 2 kind of specifiers:
+ * There are 4 kind of specifiers:
  * - co_wrapper functions can be called by only non-coroutine context, because
  *   they always generate a new coroutine.
  * - co_wrapper_mixed functions can be called by both coroutine and
  *   non-coroutine context.
+ * - co_wrapper_bdrv_rdlock are co_wrapper functions but automatically take and
+ *   release the graph rdlock when creating a new coroutine
+ * - co_wrapper_mixed_bdrv_rdlock are co_wrapper_mixed functions but
+ *   automatically take and release the graph rdlock when creating a new
+ *   coroutine.
  */
 #define co_wrapper
 #define co_wrapper_mixed
+#define co_wrapper_bdrv_rdlock
+#define co_wrapper_mixed_bdrv_rdlock
 
 #include "block/dirty-bitmap.h"
 #include "block/blockjob.h"

scripts/block-coroutine-wrapper.py

@@ -69,6 +69,7 @@ class FuncDecl:
         self.struct_name = snake_to_camel(self.name)
         self.args = [ParamDecl(arg.strip()) for arg in args.split(',')]
         self.create_only_co = 'mixed' not in variant
+        self.graph_rdlock = 'bdrv_rdlock' in variant
 
         subsystem, subname = self.name.split('_', 1)
         self.co_name = f'{subsystem}_co_{subname}'
@@ -123,10 +124,13 @@ def create_mixed_wrapper(func: FuncDecl) -> str:
     """
     name = func.co_name
     struct_name = func.struct_name
+    graph_assume_lock = 'assume_graph_lock();' if func.graph_rdlock else ''
+
     return f"""\
 {func.return_type} {func.name}({ func.gen_list('{decl}') })
 {{
     if (qemu_in_coroutine()) {{
+        {graph_assume_lock}
         return {name}({ func.gen_list('{name}') });
     }} else {{
         {struct_name} s = {{
@@ -174,6 +178,12 @@ def gen_wrapper(func: FuncDecl) -> str:
     name = func.co_name
     struct_name = func.struct_name
 
+    graph_lock=''
+    graph_unlock=''
+    if func.graph_rdlock:
+        graph_lock='    bdrv_graph_co_rdlock();'
+        graph_unlock='    bdrv_graph_co_rdunlock();'
+
     creation_function = create_mixed_wrapper
     if func.create_only_co:
         creation_function = create_co_wrapper
@@ -193,7 +203,9 @@ static void coroutine_fn {name}_entry(void *opaque)
 {{
     {struct_name} *s = opaque;
 
+{graph_lock}
     s->ret = {name}({ func.gen_list('s->{name}') });
+{graph_unlock}
     s->poll_state.in_progress = false;
 
     aio_wait_kick();