Re: [Qemu-devel] qemu_futex_wait() lockups in ARM64: 2 possible issues

[dann frazier <dann.frazier@canonical.com>]

On Fri, Oct 11, 2019 at 08:30:02AM +0000, Jan Glauber wrote:
> On Fri, Oct 11, 2019 at 10:18:18AM +0200, Paolo Bonzini wrote:
> > On 11/10/19 08:05, Jan Glauber wrote:
> > > On Wed, Oct 09, 2019 at 11:15:04AM +0200, Paolo Bonzini wrote:
> > >>> ...but if I bump notify_me size to uint64_t the issue goes away.
> > >>
> > >> Ouch. :)  Is this with or without my patch(es)?
> > 
> > You didn't answer this question.
> 
> Oh, sorry... I did but the mail probably didn't make it out.
> I have both of your changes applied (as I think they make sense).
> 
> > >> Also, what if you just add a dummy uint32_t after notify_me?
> > > 
> > > With the dummy the testcase also runs fine for 500 iterations.
> > 
> > You might be lucky and causing list_lock to be in another cache line.
> > What if you add __attribute__((aligned(16)) to notify_me (and keep the
> > dummy)?
> 
> Good point. I'll try to force both into the same cacheline.

On the Hi1620, this still hangs in the first iteration:

diff --git a/include/block/aio.h b/include/block/aio.h
index 6b0d52f732..00e56a5412 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;
+    __attribute__((aligned(16))) 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);
     }
 }