coroutine: stop using AioContext in CoQueue

qemu_co_queue_next(&queue) arranges that the next queued coroutine is
run at a later point in time.  This deferred restart is useful because
the caller may not want to transfer control yet.

This behavior was implemented using QEMUBH in the past, which meant that
CoQueue (and hence CoMutex and CoRwlock) had a dependency on the
AioContext event loop.  This hidden dependency causes trouble when we
move to a world with multiple event loops - now qemu_co_queue_next()
needs to know which event loop to schedule the QEMUBH in.

After pondering how to stash AioContext I realized the best solution is
to not use AioContext at all.  This patch implements the deferred
restart behavior purely in terms of coroutines and no longer uses
QEMUBH.

Here is how it works:

Each Coroutine has a wakeup queue that starts out empty.  When
qemu_co_queue_next() is called, the next coroutine is added to our
wakeup queue.  The wakeup queue is processed when we yield or terminate.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

diff --git a/include/block/coroutine_int.h b/include/block/coroutine_int.h
index 17eb71e7238073cdfbaf7742250d10c92eebbb78..f133d65af86d7c02d6ab375f7a3557f7f84aba78 100644 (file)
--- a/include/block/coroutine_int.h
+++ b/include/block/coroutine_int.h
@@ -38,6 +38,9 @@ struct Coroutine {
     void *entry_arg;
     Coroutine *caller;
     QSLIST_ENTRY(Coroutine) pool_next;
+
+    /* Coroutines that should be woken up when we yield or terminate */
+    QTAILQ_HEAD(, Coroutine) co_queue_wakeup;
     QTAILQ_ENTRY(Coroutine) co_queue_next;
 };
 
@@ -45,5 +48,6 @@ Coroutine *qemu_coroutine_new(void);
 void qemu_coroutine_delete(Coroutine *co);
 CoroutineAction qemu_coroutine_switch(Coroutine *from, Coroutine *to,
                                       CoroutineAction action);
+void coroutine_fn qemu_co_queue_run_restart(Coroutine *co);
 
 #endif

diff --git a/qemu-coroutine-lock.c b/qemu-coroutine-lock.c
index 86efe1f86a486da60ae03148dc521223703d7b49..d9fea4989d43294576d784a95c66990aadbde18f 100644 (file)
--- a/qemu-coroutine-lock.c
+++ b/qemu-coroutine-lock.c
@@ -26,39 +26,11 @@
 #include "block/coroutine.h"
 #include "block/coroutine_int.h"
 #include "qemu/queue.h"
-#include "block/aio.h"
 #include "trace.h"
 
-/* Coroutines are awoken from a BH to allow the current coroutine to complete
- * its flow of execution.  The BH may run after the CoQueue has been destroyed,
- * so keep BH data in a separate heap-allocated struct.
- */
-typedef struct {
-    QEMUBH *bh;
-    QTAILQ_HEAD(, Coroutine) entries;
-} CoQueueNextData;
-
-static void qemu_co_queue_next_bh(void *opaque)
-{
-    CoQueueNextData *data = opaque;
-    Coroutine *next;
-
-    trace_qemu_co_queue_next_bh();
-    while ((next = QTAILQ_FIRST(&data->entries))) {
-        QTAILQ_REMOVE(&data->entries, next, co_queue_next);
-        qemu_coroutine_enter(next, NULL);
-    }
-
-    qemu_bh_delete(data->bh);
-    g_slice_free(CoQueueNextData, data);
-}
-
 void qemu_co_queue_init(CoQueue *queue)
 {
     QTAILQ_INIT(&queue->entries);
-
-    /* This will be exposed to callers once there are multiple AioContexts */
-    queue->ctx = qemu_get_aio_context();
 }
 
 void coroutine_fn qemu_co_queue_wait(CoQueue *queue)
@@ -77,23 +49,37 @@ void coroutine_fn qemu_co_queue_wait_insert_head(CoQueue *queue)
     assert(qemu_in_coroutine());
 }
 
+/**
+ * qemu_co_queue_run_restart:
+ *
+ * Enter each coroutine that was previously marked for restart by
+ * qemu_co_queue_next() or qemu_co_queue_restart_all().  This function is
+ * invoked by the core coroutine code when the current coroutine yields or
+ * terminates.
+ */
+void qemu_co_queue_run_restart(Coroutine *co)
+{
+    Coroutine *next;
+
+    trace_qemu_co_queue_run_restart(co);
+    while ((next = QTAILQ_FIRST(&co->co_queue_wakeup))) {
+        QTAILQ_REMOVE(&co->co_queue_wakeup, next, co_queue_next);
+        qemu_coroutine_enter(next, NULL);
+    }
+}
+
 static bool qemu_co_queue_do_restart(CoQueue *queue, bool single)
 {
+    Coroutine *self = qemu_coroutine_self();
     Coroutine *next;
-    CoQueueNextData *data;
 
     if (QTAILQ_EMPTY(&queue->entries)) {
         return false;
     }
 
-    data = g_slice_new(CoQueueNextData);
-    data->bh = aio_bh_new(queue->ctx, qemu_co_queue_next_bh, data);
-    QTAILQ_INIT(&data->entries);
-    qemu_bh_schedule(data->bh);
-
     while ((next = QTAILQ_FIRST(&queue->entries)) != NULL) {
         QTAILQ_REMOVE(&queue->entries, next, co_queue_next);
-        QTAILQ_INSERT_TAIL(&data->entries, next, co_queue_next);
+        QTAILQ_INSERT_TAIL(&self->co_queue_wakeup, next, co_queue_next);
         trace_qemu_co_queue_next(next);
         if (single) {
             break;

diff --git a/qemu-coroutine.c b/qemu-coroutine.c
index 60ac79e680b65d84077af090c52627f11ed22fe4..423430d3a032b78bc69cd0032108174b1f9173f7 100644 (file)
--- a/qemu-coroutine.c
+++ b/qemu-coroutine.c
@@ -45,6 +45,7 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
     }
 
     co->entry = entry;
+    QTAILQ_INIT(&co->co_queue_wakeup);
     return co;
 }
 
@@ -87,6 +88,8 @@ static void coroutine_swap(Coroutine *from, Coroutine *to)
 
     ret = qemu_coroutine_switch(from, to, COROUTINE_YIELD);
 
+    qemu_co_queue_run_restart(to);
+
     switch (ret) {
     case COROUTINE_YIELD:
         return;

diff --git a/trace-events b/trace-events
index 9c73931f373d108e2c4ec810600a56c207c2f186..f51408aeb223f7fa43fcfbaf80294dc1cd46f680 100644 (file)
--- a/trace-events
+++ b/trace-events
@@ -825,7 +825,7 @@ qemu_coroutine_yield(void *from, void *to) "from %p to %p"
 qemu_coroutine_terminate(void *co) "self %p"
 
 # qemu-coroutine-lock.c
-qemu_co_queue_next_bh(void) ""
+qemu_co_queue_run_restart(void *co) "co %p"
 qemu_co_queue_next(void *nxt) "next %p"
 qemu_co_mutex_lock_entry(void *mutex, void *self) "mutex %p self %p"
 qemu_co_mutex_lock_return(void *mutex, void *self) "mutex %p self %p"