coroutine: abort if we try to schedule or enter a pending coroutine

The previous patch fixed a race condition, in which there were
coroutines being executing doubly, or after coroutine deletion.

We can detect common scenarios when this happens, and print an error
message and abort before we corrupt memory / data, or segfault.

This patch will abort if an attempt to enter a coroutine is made while
it is currently pending execution, either in a specific AioContext bh,
or pending execution via a timer.  It will also abort if a coroutine
is scheduled, before a prior scheduled run has occurred.

We cannot rely on the existing co->caller check for recursive re-entry
to catch this, as the coroutine may run and exit with
COROUTINE_TERMINATE before the scheduled coroutine executes.

(This is the scenario that was occurring and fixed in the previous
patch).

This patch also re-orders the Coroutine struct elements in an attempt to
optimize caching.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
index d6095c1d5a..9eff7fd450 100644
--- a/util/qemu-coroutine.c
+++ b/util/qemu-coroutine.c
@@ -107,8 +107,22 @@ void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co)
     Coroutine *self = qemu_coroutine_self();
     CoroutineAction ret;
 
+    /* Cannot rely on the read barrier for co in aio_co_wake(), as there are
+     * callers outside of aio_co_wake() */
+    const char *scheduled = atomic_mb_read(&co->scheduled);
+
     trace_qemu_aio_coroutine_enter(ctx, self, co, co->entry_arg);
 
+    /* if the Coroutine has already been scheduled, entering it again will
+     * cause us to enter it twice, potentially even after the coroutine has
+     * been deleted */
+    if (scheduled) {
+        fprintf(stderr,
+                "%s: Co-routine was already scheduled in '%s'\n",
+                __func__, scheduled);
+        abort();
+    }
+
     if (co->caller) {
         fprintf(stderr, "Co-routine re-entered recursively\n");
         abort();