qemu/async-use-explicit-memory-barriers.patch

From: Paolo Bonzini <pbonzini@redhat.com>
Date: Tue, 7 Apr 2020 10:07:46 -0400
Subject: async: use explicit memory barriers

Git-commit: 5710a3e09f9b85801e5ce70797a4a511e5fc9e2c

When using C11 atomics, non-seqcst reads and writes do not participate
in the total order of seqcst operations.  In util/async.c and util/aio-posix.c,
in particular, the pattern that we use

          write ctx->notify_me                 write bh->scheduled
          read bh->scheduled                   read ctx->notify_me
          if !bh->scheduled, sleep             if ctx->notify_me, notify

needs to use seqcst operations for both the write and the read.  In
general this is something that we do not want, because there can be
many sources that are polled in addition to bottom halves.  The
alternative is to place a seqcst memory barrier between the write
and the read.  This also comes with a disadvantage, in that the
memory barrier is implicit on strongly-ordered architectures and
it wastes a few dozen clock cycles.

Fortunately, ctx->notify_me is never written concurrently by two
threads, so we can assert that and relax the writes to ctx->notify_me.
The resulting solution works and performs well on both aarch64 and x86.

Note that the atomic_set/atomic_read combination is not an atomic
read-modify-write, and therefore it is even weaker than C11 ATOMIC_RELAXED;
on x86, ATOMIC_RELAXED compiles to a locked operation.

Analyzed-by: Ying Fang <fangying1@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Ying Fang <fangying1@huawei.com>
Message-Id: <20200407140746.8041-6-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Bruce Rogers <brogers@suse.com>
---
 util/aio-posix.c | 16 ++++++++++++++--
 util/aio-win32.c | 17 ++++++++++++++---
 util/async.c     | 16 ++++++++++++----
 3 files changed, 40 insertions(+), 9 deletions(-)

diff --git a/util/aio-posix.c b/util/aio-posix.c
index a4977f538ef28d56178267a1795c..fe2a46c439fa1505f5f688274566 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -616,6 +616,11 @@ bool aio_poll(AioContext *ctx, bool blocking)
     int64_t timeout;
     int64_t start = 0;
 
+    /*
+     * There cannot be two concurrent aio_poll calls for the same AioContext (or
+     * an aio_poll concurrent with a GSource prepare/check/dispatch callback).
+     * We rely on this below to avoid slow locked accesses to ctx->notify_me.
+     */
     assert(in_aio_context_home_thread(ctx));
 
     /* aio_notify can avoid the expensive event_notifier_set if
@@ -626,7 +631,13 @@ bool aio_poll(AioContext *ctx, bool blocking)
      * so disable the optimization now.
      */
     if (blocking) {
-        atomic_add(&ctx->notify_me, 2);
+        atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
+        /*
+         * Write ctx->notify_me before computing the timeout
+         * (reading bottom half flags, etc.).  Pairs with
+         * smp_mb in aio_notify().
+         */
+        smp_mb();
     }
 
     qemu_lockcnt_inc(&ctx->list_lock);
@@ -671,7 +682,8 @@ bool aio_poll(AioContext *ctx, bool blocking)
     }
 
     if (blocking) {
-        atomic_sub(&ctx->notify_me, 2);
+        /* Finish the poll before clearing the flag.  */
+        atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me) - 2);
         aio_notify_accept(ctx);
     }
 
diff --git a/util/aio-win32.c b/util/aio-win32.c
index a23b9c364db3a764a3e00c6b62e9..729d533faf4d807e0a5388edd2af 100644
--- a/util/aio-win32.c
+++ b/util/aio-win32.c
@@ -321,6 +321,12 @@ bool aio_poll(AioContext *ctx, bool blocking)
     int count;
     int timeout;
 
+    /*
+     * There cannot be two concurrent aio_poll calls for the same AioContext (or
+     * an aio_poll concurrent with a GSource prepare/check/dispatch callback).
+     * We rely on this below to avoid slow locked accesses to ctx->notify_me.
+     */
+    assert(in_aio_context_home_thread(ctx));
     progress = false;
 
     /* aio_notify can avoid the expensive event_notifier_set if
@@ -331,7 +337,13 @@ bool aio_poll(AioContext *ctx, bool blocking)
      * so disable the optimization now.
      */
     if (blocking) {
-        atomic_add(&ctx->notify_me, 2);
+        atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
+        /*
+         * Write ctx->notify_me before computing the timeout
+         * (reading bottom half flags, etc.).  Pairs with
+         * smp_mb in aio_notify().
+         */
+        smp_mb();
     }
 
     qemu_lockcnt_inc(&ctx->list_lock);
@@ -364,8 +376,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
         ret = WaitForMultipleObjects(count, events, FALSE, timeout);
         if (blocking) {
             assert(first);
-            assert(in_aio_context_home_thread(ctx));
-            atomic_sub(&ctx->notify_me, 2);
+            atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me) - 2);
             aio_notify_accept(ctx);
         }
 
diff --git a/util/async.c b/util/async.c
index b1fa5319e5bc7830d50108f91139..c65c58bbc9f57bf1bbdb6acd5fd1 100644
--- a/util/async.c
+++ b/util/async.c
@@ -220,7 +220,14 @@ aio_ctx_prepare(GSource *source, gint    *timeout)
 {
     AioContext *ctx = (AioContext *) source;
 
-    atomic_or(&ctx->notify_me, 1);
+    atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) | 1);
+
+    /*
+     * Write ctx->notify_me before computing the timeout
+     * (reading bottom half flags, etc.).  Pairs with
+     * smp_mb in aio_notify().
+     */
+    smp_mb();
 
     /* We assume there is no timeout already supplied */
     *timeout = qemu_timeout_ns_to_ms(aio_compute_timeout(ctx));
@@ -238,7 +245,8 @@ aio_ctx_check(GSource *source)
     AioContext *ctx = (AioContext *) source;
     QEMUBH *bh;
 
-    atomic_and(&ctx->notify_me, ~1);
+    /* Finish computing the timeout before clearing the flag.  */
+    atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me) & ~1);
     aio_notify_accept(ctx);
 
     for (bh = ctx->first_bh; bh; bh = bh->next) {
@@ -343,10 +351,10 @@ 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.
+     * with smp_mb in aio_ctx_prepare or aio_poll.
      */
     smp_mb();
-    if (ctx->notify_me) {
+    if (atomic_read(&ctx->notify_me)) {
         event_notifier_set(&ctx->notifier);
         atomic_mb_set(&ctx->notified, true);
     }