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[Qemu-devel] [PATCH for-2.11 3/4] coroutine: Cancel aio_co_schedule() on

From: Kevin Wolf
Subject: [Qemu-devel] [PATCH for-2.11 3/4] coroutine: Cancel aio_co_schedule() on direct entry
Date: Tue, 28 Nov 2017 16:43:49 +0100

If a coroutine can be reentered from multiple possible sources, we need
to be careful in the case that two sources try to reenter it at the same

There are two different cases where this can happen:

1. A coroutine spawns multiple asynchronous jobs and waits for all of
   them to complete. In this case, multiple reentries are expected and
   the coroutine is probably looping around a yield, so entering it
   twice is generally fine (but entering it just once after all jobs
   have completed would be enough, too).

   Exception: When the loop condition becomes false and the first
   reenter already leaves the loop, we must not do a second reenter.

2. A coroutine yields and provides multiple alternative ways to be
   reentered. In this case, we must always only process the first

Direct entry is not a problem. It requires that the AioContext locks for
the coroutine context are held, which means that the coroutine has
enough time to set some state that simply prevents the second caller
from reentering the coroutine, too.

Things get more interesting with aio_co_schedule() because the coroutine
may be scheduled before it is (directly) entered from a second place.
Then changing some state inside the coroutine doesn't help because it is
already scheduled and the state won't be checked again.

For this case, we'll cancel any pending aio_co_schedule() when the
coroutine is actually entered. Reentering it once is enough for all
cases explained above, and a requirement for part of them.

Signed-off-by: Kevin Wolf <address@hidden>
 include/qemu/coroutine_int.h |  1 +
 util/async.c                 | 15 ++++++++++++++-
 util/qemu-coroutine.c        | 17 +++++++++++++++++
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/include/qemu/coroutine_int.h b/include/qemu/coroutine_int.h
index cb98892bba..73fc35e54b 100644
--- a/include/qemu/coroutine_int.h
+++ b/include/qemu/coroutine_int.h
@@ -40,6 +40,7 @@ struct Coroutine {
     CoroutineEntry *entry;
     void *entry_arg;
     Coroutine *caller;
+    AioContext *scheduled;
     /* Only used when the coroutine has terminated.  */
     QSLIST_ENTRY(Coroutine) pool_next;
diff --git a/util/async.c b/util/async.c
index 0e1bd8780a..dc249e9404 100644
--- a/util/async.c
+++ b/util/async.c
@@ -372,6 +372,7 @@ static bool event_notifier_poll(void *opaque)
 static void co_schedule_bh_cb(void *opaque)
     AioContext *ctx = opaque;
+    AioContext *scheduled_ctx;
     QSLIST_HEAD(, Coroutine) straight, reversed;
     QSLIST_MOVE_ATOMIC(&reversed, &ctx->scheduled_coroutines);
@@ -388,7 +389,16 @@ static void co_schedule_bh_cb(void *opaque)
         QSLIST_REMOVE_HEAD(&straight, co_scheduled_next);
         trace_aio_co_schedule_bh_cb(ctx, co);
-        qemu_coroutine_enter(co);
+        scheduled_ctx = atomic_mb_read(&co->scheduled);
+        if (scheduled_ctx == ctx) {
+            qemu_coroutine_enter(co);
+        } else {
+            /* This must be a cancelled aio_co_schedule() because the coroutine
+             * was already entered before this BH had a chance to run. If a
+             * coroutine is scheduled for two different AioContexts, something
+             * is very wrong. */
+            assert(scheduled_ctx == NULL);
+        }
@@ -438,6 +448,9 @@ fail:
 void aio_co_schedule(AioContext *ctx, Coroutine *co)
     trace_aio_co_schedule(ctx, co);
+    /* Set co->scheduled before the coroutine becomes visible in the list */
+    atomic_mb_set(&co->scheduled, ctx);
                               co, co_scheduled_next);
diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
index d6095c1d5a..c515b3cb4a 100644
--- a/util/qemu-coroutine.c
+++ b/util/qemu-coroutine.c
@@ -122,6 +122,23 @@ void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine 
+    /* Make sure that a coroutine that can alternatively reentered from two
+     * different sources isn't reentered more than once when the first caller
+     * uses aio_co_schedule() and the other one enters to coroutine directly.
+     * This is achieved by cancelling the pending aio_co_schedule().
+     *
+     * The other way round, if aio_co_schedule() would be called after this
+     * point, this would be a problem, too, but in practice it doesn't happen
+     * because we're holding the AioContext lock here and aio_co_schedule()
+     * callers must do the same. This means that the coroutine just needs to
+     * prevent other callers from calling aio_co_schedule() before it yields
+     * (e.g. block job coroutines by setting job->busy = true).
+     *
+     * We still want to ensure that the second case doesn't happen, so reset
+     * co->scheduled only after setting co->caller to make the above check
+     * effective for the co_schedule_bh_cb() case. */
+    atomic_set(&co->scheduled, NULL);
     ret = qemu_coroutine_switch(self, co, COROUTINE_ENTER);

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