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

[Paolo Bonzini]

On 02/10/19 11:23, Jan Glauber wrote:
> I've looked into this on ThunderX2. The arm64 code generated for the
> atomic_[add|sub] accesses of ctx->notify_me doesn't contain any
> memory barriers. It is just plain ldaxr/stlxr.
> 
> From my understanding this is not sufficient for SMP sync.
> 
> If I read this comment correct:
> 
>     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.
>          */
>         smp_mb();
>         if (ctx->notify_me) {
> 
> it points out that the smp_mb() should be paired. But as
> I said the used atomics don't generate any barriers at all.

Based on the rest of the thread, this patch should also fix the bug:

diff --git a/util/async.c b/util/async.c
index 47dcbfa..721ea53 100644
--- a/util/async.c
+++ b/util/async.c
@@ -249,7 +249,7 @@ 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;
         }
     }


And also the memory barrier in aio_notify can actually be replaced
with a SEQ_CST load:

diff --git a/util/async.c b/util/async.c
index 47dcbfa..721ea53 100644
--- a/util/async.c
+++ b/util/async.c
@@ -349,11 +349,11 @@ 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) {
+    if (atomic_mb_read(&ctx->notify_me)) {
         event_notifier_set(&ctx->notifier);
         atomic_mb_set(&ctx->notified, true);
     }


Would you be able to test these (one by one possibly)?

> I've tried to verify me theory with this patch and didn't run into the
> issue for ~500 iterations (usually I would trigger the issue ~20 iterations).

Sorry for asking the obvious---500 iterations of what?

Paolo