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>

include/qemu/coroutine_int.h

@@ -46,14 +46,21 @@ struct Coroutine {
 
     size_t locks_held;
 
+    /* Only used when the coroutine has yielded.  */
+    AioContext *ctx;
+
+    /* Used to catch and abort on illegal co-routine entry.
+     * Will contain the name of the function that had first
+     * scheduled the coroutine. */
+    const char *scheduled;
+
+    QSIMPLEQ_ENTRY(Coroutine) co_queue_next;
+
     /* Coroutines that should be woken up when we yield or terminate.
      * Only used when the coroutine is running.
      */
     QSIMPLEQ_HEAD(, Coroutine) co_queue_wakeup;
 
-    /* Only used when the coroutine has yielded.  */
-    AioContext *ctx;
-    QSIMPLEQ_ENTRY(Coroutine) co_queue_next;
     QSLIST_ENTRY(Coroutine) co_scheduled_next;
 };
 

util/async.c

@@ -388,6 +388,9 @@ static void co_schedule_bh_cb(void *opaque)
         QSLIST_REMOVE_HEAD(&straight, co_scheduled_next);
         trace_aio_co_schedule_bh_cb(ctx, co);
         aio_context_acquire(ctx);
+
+        /* Protected by write barrier in qemu_aio_coroutine_enter */
+        atomic_set(&co->scheduled, NULL);
         qemu_coroutine_enter(co);
         aio_context_release(ctx);
     }
@@ -438,6 +441,16 @@ fail:
 void aio_co_schedule(AioContext *ctx, Coroutine *co)
 {
     trace_aio_co_schedule(ctx, co);
+    const char *scheduled = atomic_cmpxchg(&co->scheduled, NULL,
+                                           __func__);
+
+    if (scheduled) {
+        fprintf(stderr,
+                "%s: Co-routine was already scheduled in '%s'\n",
+                __func__, scheduled);
+        abort();
+    }
+
     QSLIST_INSERT_HEAD_ATOMIC(&ctx->scheduled_coroutines,
                               co, co_scheduled_next);
     qemu_bh_schedule(ctx->co_schedule_bh);

util/qemu-coroutine-sleep.c

@@ -13,6 +13,7 @@
 
 #include "qemu/osdep.h"
 #include "qemu/coroutine.h"
+#include "qemu/coroutine_int.h"
 #include "qemu/timer.h"
 #include "block/aio.h"
 
@@ -25,6 +26,8 @@ static void co_sleep_cb(void *opaque)
 {
     CoSleepCB *sleep_cb = opaque;
 
+    /* Write of schedule protected by barrier write in aio_co_schedule */
+    atomic_set(&sleep_cb->co->scheduled, NULL);
     aio_co_wake(sleep_cb->co);
 }
 
@@ -34,6 +37,15 @@ void coroutine_fn co_aio_sleep_ns(AioContext *ctx, QEMUClockType type,
     CoSleepCB sleep_cb = {
         .co = qemu_coroutine_self(),
     };
+
+    const char *scheduled = atomic_cmpxchg(&sleep_cb.co->scheduled, NULL,
+                                           __func__);
+    if (scheduled) {
+        fprintf(stderr,
+                "%s: Co-routine was already scheduled in '%s'\n",
+                __func__, scheduled);
+        abort();
+    }
     sleep_cb.ts = aio_timer_new(ctx, type, SCALE_NS, co_sleep_cb, &sleep_cb);
     timer_mod(sleep_cb.ts, qemu_clock_get_ns(type) + ns);
     qemu_coroutine_yield();

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();