Comment 32 for bug 1805256

On Fri, Oct 11, 2019 at 06:05:25AM +0000, Jan Glauber wrote:
> On Wed, Oct 09, 2019 at 11:15:04AM +0200, Paolo Bonzini wrote:
> > On 09/10/19 10:02, Jan Glauber wrote:
>
> > > I'm still not sure what the actual issue is here, but could it be some bad
> > > interaction between the notify_me and the list_lock? The are both 4 byte
> > > and side-by-side:
> > >
> > > address notify_me: 0xaaaadb528aa0 sizeof notify_me: 4
> > > address list_lock: 0xaaaadb528aa4 sizeof list_lock: 4
> > >
> > > AFAICS the generated code looks OK (all load/store exclusive done
> > > with 32 bit size):
> > >
> > > e6c: 885ffc01 ldaxr w1, [x0]
> > > e70: 11000821 add w1, w1, #0x2
> > > e74: 8802fc01 stlxr w2, w1, [x0]
> > >
> > > ...but if I bump notify_me size to uint64_t the issue goes away.
> >
> > Ouch. :) Is this with or without my patch(es)?
> >
> > Also, what if you just add a dummy uint32_t after notify_me?
>
> With the dummy the testcase also runs fine for 500 iterations.
>
> Dann, can you try if this works on the Hi1620 too?

On Hi1620, it hung on the first iteration. Here's the complete patch
I'm running with:

diff --git a/include/block/aio.h b/include/block/aio.h
index 6b0d52f732..e6fd6f1a1a 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -82,7 +82,7 @@ struct AioContext {
      * Instead, the aio_poll calls include both the prepare and the
      * dispatch phase, hence a simple counter is enough for them.
      */
- uint32_t notify_me;
+ uint64_t notify_me;

     /* A lock to protect between QEMUBH and AioHandler adders and deleter,
      * and to ensure that no callbacks are removed while we're walking and
diff --git a/util/async.c b/util/async.c
index ca83e32c7f..024c4c567d 100644
--- a/util/async.c
+++ b/util/async.c
@@ -242,7 +242,7 @@ aio_ctx_check(GSource *source)
     aio_notify_accept(ctx);

     for (bh = ctx->first_bh; bh; bh = bh->next) {
- if (bh->scheduled) {
+ if (atomic_mb_read(&bh->scheduled)) {
             return true;
         }
     }
@@ -342,12 +342,12 @@ LinuxAioState *aio_get_linux_aio(AioContext *ctx)

 void aio_notify(AioContext *ctx)
 {
- /* Write e.g. bh->scheduled before reading ctx->notify_me. Pairs
- * with atomic_or in aio_ctx_prepare or atomic_add in aio_poll.
+ /* Using atomic_mb_read ensures that e.g. bh->scheduled is written before
+ * ctx->notify_me is read. Pairs with atomic_or in aio_ctx_prepare or
+ * atomic_add in aio_poll.
      */
- smp_mb();
- if (ctx->notify_me) {
- event_notifier_set(&ctx->notifier);
+ if (atomic_mb_read(&ctx->notify_me)) {
+ event_notifier_set(&ctx->notifier);
         atomic_mb_set(&ctx->notified, true);
     }
 }