[Xen-devel] [PATCH v2] libxl: Introduce nested async operations (nested ao)

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Roger Pau Monne writes ("[PATCH v2 01/10] libxl: Introduce nested async 
operations (nested ao)"):
> This allows a long-running ao to avoid accumulating memory.  Each
> nested ao has its own gc.

Thanks.  Ian C wanted an improved comment, so:

From: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Date: Mon, 4 Nov 2013 17:56:15 +0000
Subject: [PATCH] libxl: Introduce nested async operations (nested ao)
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This allows a long-running ao to avoid accumulating memory.  Each
nested ao has its own gc.

Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>

---
v2: Mention in comment that multiple levels of nesting is OK.
---
 tools/libxl/libxl_event.c    |   35 +++++++++++++++++++++++++++++++++++
 tools/libxl/libxl_internal.h |   25 ++++++++++++++++++++++++-
 2 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c
index 0fe4428..54d15db 100644
--- a/tools/libxl/libxl_event.c
+++ b/tools/libxl/libxl_event.c
@@ -1572,6 +1572,7 @@ void libxl__ao_complete(libxl__egc *egc, libxl__ao *ao, 
int rc)
     LOG(DEBUG,"ao %p: complete, rc=%d",ao,rc);
     assert(ao->magic == LIBXL__AO_MAGIC);
     assert(!ao->complete);
+    assert(!ao->nested);
     ao->complete = 1;
     ao->rc = rc;
 
@@ -1736,6 +1737,7 @@ void libxl__ao_progress_report(libxl__egc *egc, libxl__ao 
*ao,
         const libxl_asyncprogress_how *how, libxl_event *ev)
 {
     AO_GC;
+    assert(!ao->nested);
     if (how->callback == dummy_asyncprogress_callback_ignore) {
         LOG(DEBUG,"ao %p: progress report: ignored",ao);
         libxl_event_free(CTX,ev);
@@ -1756,6 +1758,39 @@ void libxl__ao_progress_report(libxl__egc *egc, 
libxl__ao *ao,
 }
 
 
+/* nested ao */
+
+_hidden libxl__ao *libxl__nested_ao_create(libxl__ao *parent)
+{
+    /* We only use the parent to get the ctx.  However, we require the
+     * caller to provide us with an ao, not just a ctx, to prove that
+     * they are already in an asynchronous operation.  That will avoid
+     * people using this to (for example) make an ao in a non-ao_how
+     * function somewhere in the middle of libxl. */
+    libxl__ao *child = NULL;
+    libxl_ctx *ctx = libxl__gc_owner(&parent->gc);
+
+    assert(parent->magic == LIBXL__AO_MAGIC);
+
+    child = libxl__zalloc(&ctx->nogc_gc, sizeof(*child));
+    child->magic = LIBXL__AO_MAGIC;
+    child->nested = 1;
+    LIBXL_INIT_GC(child->gc, ctx);
+    libxl__gc *gc = &child->gc;
+
+    LOG(DEBUG,"ao %p: nested ao, parent %p", child, parent);
+    return child;
+}
+
+_hidden void libxl__nested_ao_free(libxl__ao *child)
+{
+    assert(child->magic == LIBXL__AO_MAGIC);
+    assert(child->nested);
+    libxl_ctx *ctx = libxl__gc_owner(&child->gc);
+    libxl__ao__destroy(ctx, child);
+}
+
+
 /*
  * Local variables:
  * mode: C
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 4729566..0b20434 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -427,7 +427,7 @@ struct libxl__ao {
      * only in libxl__ao_complete.)
      */
     uint32_t magic;
-    unsigned constructing:1, in_initiator:1, complete:1, notified:1;
+    unsigned constructing:1, in_initiator:1, complete:1, notified:1, nested:1;
     int progress_reports_outstanding;
     int rc;
     libxl__gc gc;
@@ -1780,6 +1780,29 @@ _hidden void 
libxl__ao_complete_check_progress_reports(libxl__egc*, libxl__ao*);
 
 
 /*
+ * Short-lived sub-ao, aka "nested ao".
+ *
+ * Some asynchronous operations are very long-running.  Generally,
+ * since an ao has a gc, any allocations made in that ao will live
+ * until the ao is completed.  When this is not desirable, these
+ * functions may be used to manage a "sub-ao".
+ *
+ * The returned sub-ao is suitable for passing to gc-related functions
+ * and macros such as libxl__ao_inprogress_gc, AO_GC, and STATE_AO_GC.
+ *
+ * It MUST NOT be used with AO_INPROGRESS, AO_ABORT,
+ * libxl__ao_complete, libxl__ao_progress_report, and so on.
+ *
+ * The caller must ensure that all of the sub-ao's are freed before
+ * the parent is.  Multiple levels of nesting are OK (although
+ * hopefully they won't be necessary).
+ */
+
+_hidden libxl__ao *libxl__nested_ao_create(libxl__ao *parent); /* cannot fail 
*/
+_hidden void libxl__nested_ao_free(libxl__ao *child);
+
+
+/*
  * File descriptors and CLOEXEC
  */
 
-- 
1.7.10.4


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