[Qemu-devel] [PATCH 7/9] tcg: synchronize exit_request and tcg_current_cpu accesses

[Paolo Bonzini <pbonzini@redhat.com>]

Synchronize the remaining pair of accesses in cpu_signal.  The
wrongly-ordered accesses in cpu_signal are currently not an issue on
Windows because they execute atomically between SuspendProcess and
ResumeProcess.  Only cpu_exec can be split (and the newly introduced
atomic_mb_read would be needed on Windows too, but the compiler
must not be doing strange optimizations).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 cpu-exec.c |  2 +-
 cpus.c     | 14 ++++++++++----
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/cpu-exec.c b/cpu-exec.c
index 2128bf1..b337506 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -409,7 +409,7 @@ int cpu_exec(CPUState *cpu)
     atomic_mb_set(&tcg_current_cpu, cpu);
     rcu_read_lock();
 
-    if (unlikely(exit_request)) {
+    if (unlikely(atomic_mb_read(&exit_request))) {
         cpu->exit_request = 1;
     }
 
diff --git a/cpus.c b/cpus.c
index ec8168c..783ef00 100644
--- a/cpus.c
+++ b/cpus.c
@@ -663,11 +663,15 @@ static void cpu_handle_guest_debug(CPUState *cpu)
 
 static void cpu_signal(int sig)
 {
-    CPUState *cpu = atomic_mb_read(&tcg_current_cpu);
+    CPUState *cpu;
+    /* Ensure whatever caused the exit has reached the CPU threads before
+     * writing exit_request.
+     */
+    atomic_mb_set(&exit_request, 1);
+    cpu = atomic_mb_read(&tcg_current_cpu);
     if (cpu) {
         cpu_exit(cpu);
     }
-    exit_request = 1;
 }
 
 #ifdef CONFIG_LINUX
@@ -1074,7 +1078,7 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
     }
 
     /* process any pending work */
-    exit_request = 1;
+    atomic_mb_set(&exit_request, 1);
 
     while (1) {
         tcg_exec_all();
@@ -1453,7 +1457,9 @@ static void tcg_exec_all(void)
             break;
         }
     }
-    exit_request = 0;
+
+    /* Pairs with smp_wmb in qemu_cpu_kick.  */
+    atomic_mb_set(&exit_request, 0);
 }
 
 void list_cpus(FILE *f, fprintf_function cpu_fprintf, const char *optarg)
-- 
2.4.3