[Qemu-devel] [PATCH 1/2] AioContext: fix broken placement of event_notifier_test_and_clear

[Paolo Bonzini <pbonzini@redhat.com>]

event_notifier_test_and_clear must be called before processing events.
Otherwise, an aio_poll could "eat" the notification before the main
I/O thread invokes ppoll().  The main I/O thread then never wakes up.
This is an example of what could happen:

   i/o thread         vcpu thread                   worker thread
   ---------------------------------------------------------------------
   lock_iothread
   notify_me = 1
   ...
   unlock_iothread
                      lock_iothread
                      notify_me = 3
                      ppoll
                      notify_me = 1
                                                     bh->scheduled = 1
                                                     event_notifier_set
                      event_notifier_test_and_clear
   ppoll
   *** hang ***

In contrast with the previous bug, there wasn't really much debugging
to do here.  "Tracing" with qemu_clock_get_ns shows pretty much the
same behavior as in the previous patch, so the only wait to find the
bug is staring at the code.

One could also use a formal model, of course.  The included one shows
this with three processes: notifier corresponds to a QEMU thread pool
worker, temporary_waiter to a VCPU thread that invokes aio_poll(),
waiter to the main I/O thread.  I would be happy to say that the
formal model found the bug for me, but actually I wrote it after the
fact.

This patch is a bit of a big hammer.  The next one optimizes it,
with help (this time for real rather than a posteriori :)) from
another, similar formal model.

Reported-by: Richard W. M. Jones <rjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 aio-posix.c                 |   2 +
 aio-win32.c                 |   2 +
 async.c                     |   8 ++-
 docs/aio_notify_bug.promela | 140 ++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 151 insertions(+), 1 deletion(-)
 create mode 100644 docs/aio_notify_bug.promela

diff --git a/aio-posix.c b/aio-posix.c
index 249889f..5c8b266 100644
--- a/aio-posix.c
+++ b/aio-posix.c
@@ -276,6 +276,8 @@ bool aio_poll(AioContext *ctx, bool blocking)
         aio_context_acquire(ctx);
     }
 
+    event_notifier_test_and_clear(&ctx->notifier);
+
     /* if we have any readable fds, dispatch event */
     if (ret > 0) {
         for (i = 0; i < npfd; i++) {
diff --git a/aio-win32.c b/aio-win32.c
index ea655b0..3e0db20 100644
--- a/aio-win32.c
+++ b/aio-win32.c
@@ -337,6 +337,8 @@ bool aio_poll(AioContext *ctx, bool blocking)
             aio_context_acquire(ctx);
         }
 
+        event_notifier_test_and_clear(&ctx->notifier);
+
         if (first && aio_bh_poll(ctx)) {
             progress = true;
         }
diff --git a/async.c b/async.c
index a232192..d625e8a 100644
--- a/async.c
+++ b/async.c
@@ -203,6 +203,8 @@ aio_ctx_check(GSource *source)
     QEMUBH *bh;
 
     atomic_and(&ctx->notify_me, ~1);
+    event_notifier_test_and_clear(&ctx->notifier);
+
     for (bh = ctx->first_bh; bh; bh = bh->next) {
         if (!bh->deleted && bh->scheduled) {
             return true;
@@ -279,6 +281,10 @@ static void aio_rfifolock_cb(void *opaque)
     aio_notify(opaque);
 }
 
+static void event_notifier_dummy_cb(EventNotifier *e)
+{
+}
+
 AioContext *aio_context_new(Error **errp)
 {
     int ret;
@@ -293,7 +299,7 @@ AioContext *aio_context_new(Error **errp)
     g_source_set_can_recurse(&ctx->source, true);
     aio_set_event_notifier(ctx, &ctx->notifier,
                            (EventNotifierHandler *)
-                           event_notifier_test_and_clear);
+                           event_notifier_dummy_cb);
     ctx->thread_pool = NULL;
     qemu_mutex_init(&ctx->bh_lock);
     rfifolock_init(&ctx->lock, aio_rfifolock_cb, ctx);
diff --git a/docs/aio_notify_bug.promela b/docs/aio_notify_bug.promela
new file mode 100644
index 0000000..b3bfca1
--- /dev/null
+++ b/docs/aio_notify_bug.promela
@@ -0,0 +1,140 @@
+/*
+ * This model describes a bug in aio_notify.  If ctx->notifier is
+ * cleared too late, a wakeup could be lost.
+ *
+ * Author: Paolo Bonzini <pbonzini@redhat.com>
+ *
+ * This file is in the public domain.  If you really want a license,
+ * the WTFPL will do.
+ *
+ * To verify the buggy version:
+ *     spin -a -DBUG docs/aio_notify_bug.promela
+ *     gcc -O2 pan.c
+ *     ./a.out -a -f
+ *
+ * To verify the working version:
+ *     spin -a docs/aio_notify_bug.promela
+ *     gcc -O2 pan.c
+ *     ./a.out -a -f
+ *
+ * Add -DCHECK_REQ to test an alternative invariant and the
+ * "notify_me" optimization.
+ */
+
+int notify_me;
+bool event;
+bool req;
+bool notifier_done;
+
+#ifdef CHECK_REQ
+#define USE_NOTIFY_ME 1
+#else
+#define USE_NOTIFY_ME 0
+#endif
+
+active proctype notifier()
+{
+    do
+        :: true -> {
+            req = 1;
+            if
+               :: !USE_NOTIFY_ME || notify_me -> event = 1;
+               :: else -> skip;
+            fi
+        }
+        :: true -> break;
+    od;
+    notifier_done = 1;
+}
+
+#ifdef BUG
+#define AIO_POLL                                                    \
+    notify_me++;                                                    \
+    if                                                              \
+        :: !req -> {                                                \
+            if                                                      \
+                :: event -> skip;                                   \
+            fi;                                                     \
+        }                                                           \
+        :: else -> skip;                                            \
+    fi;                                                             \
+    notify_me--;                                                    \
+                                                                    \
+    req = 0;                                                        \
+    event = 0;
+#else
+#define AIO_POLL                                                    \
+    notify_me++;                                                    \
+    if                                                              \
+        :: !req -> {                                                \
+            if                                                      \
+                :: event -> skip;                                   \
+            fi;                                                     \
+        }                                                           \
+        :: else -> skip;                                            \
+    fi;                                                             \
+    notify_me--;                                                    \
+                                                                    \
+    event = 0;                                                      \
+    req = 0;
+#endif
+
+active proctype waiter()
+{
+    do
+       :: true -> AIO_POLL;
+    od;
+}
+
+/* Same as waiter(), but disappears after a while.  */
+active proctype temporary_waiter()
+{
+    do
+       :: true -> AIO_POLL;
+       :: true -> break;
+    od;
+}
+
+#ifdef CHECK_REQ
+never {
+    do
+        :: req -> goto accept_if_req_not_eventually_false;
+        :: true -> skip;
+    od;
+
+accept_if_req_not_eventually_false:
+    if
+        :: req -> goto accept_if_req_not_eventually_false;
+    fi;
+    assert(0);
+}
+
+#else
+/* There must be infinitely many transitions of event as long
+ * as the notifier does not exit.
+ *
+ * If event stayed always true, the waiters would be busy looping.
+ * If event stayed always false, the waiters would be sleeping
+ * forever.
+ */
+never {
+    do
+        :: !event    -> goto accept_if_event_not_eventually_true;
+        :: event     -> goto accept_if_event_not_eventually_false;
+        :: true      -> skip;
+    od;
+
+accept_if_event_not_eventually_true:
+    if
+        :: !event && notifier_done  -> do :: true -> skip; od;
+        :: !event && !notifier_done -> goto accept_if_event_not_eventually_true;
+    fi;
+    assert(0);
+
+accept_if_event_not_eventually_false:
+    if
+        :: event     -> goto accept_if_event_not_eventually_false;
+    fi;
+    assert(0);
+}
+#endif
-- 
2.4.3