cpus-common: Introduce async_safe_run_on_cpu()

Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

cpus-common.c

@@ -18,6 +18,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/main-loop.h"
 #include "exec/cpu-common.h"
 #include "qom/cpu.h"
 #include "sysemu/cpus.h"
@@ -106,7 +107,7 @@ struct qemu_work_item {
     struct qemu_work_item *next;
     run_on_cpu_func func;
     void *data;
-    bool free, done;
+    bool free, exclusive, done;
 };
 
 static void queue_work_on_cpu(CPUState *cpu, struct qemu_work_item *wi)
@@ -139,6 +140,7 @@ void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data,
     wi.data = data;
     wi.done = false;
     wi.free = false;
+    wi.exclusive = false;
 
     queue_work_on_cpu(cpu, &wi);
     while (!atomic_mb_read(&wi.done)) {
@@ -229,6 +231,19 @@ void cpu_exec_end(CPUState *cpu)
     qemu_mutex_unlock(&qemu_cpu_list_lock);
 }
 
+void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data)
+{
+    struct qemu_work_item *wi;
+
+    wi = g_malloc0(sizeof(struct qemu_work_item));
+    wi->func = func;
+    wi->data = data;
+    wi->free = true;
+    wi->exclusive = true;
+
+    queue_work_on_cpu(cpu, wi);
+}
+
 void process_queued_cpu_work(CPUState *cpu)
 {
     struct qemu_work_item *wi;
@@ -245,7 +260,21 @@ void process_queued_cpu_work(CPUState *cpu)
             cpu->queued_work_last = NULL;
         }
         qemu_mutex_unlock(&cpu->work_mutex);
-        wi->func(cpu, wi->data);
+        if (wi->exclusive) {
+            /* Running work items outside the BQL avoids the following deadlock:
+             * 1) start_exclusive() is called with the BQL taken while another
+             * CPU is running; 2) cpu_exec in the other CPU tries to takes the
+             * BQL, so it goes to sleep; start_exclusive() is sleeping too, so
+             * neither CPU can proceed.
+             */
+            qemu_mutex_unlock_iothread();
+            start_exclusive();
+            wi->func(cpu, wi->data);
+            end_exclusive();
+            qemu_mutex_lock_iothread();
+        } else {
+            wi->func(cpu, wi->data);
+        }
         qemu_mutex_lock(&cpu->work_mutex);
         if (wi->free) {
             g_free(wi);

include/qom/cpu.h

@@ -655,6 +655,20 @@ void run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data);
  */
 void async_run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data);
 
+/**
+ * async_safe_run_on_cpu:
+ * @cpu: The vCPU to run on.
+ * @func: The function to be executed.
+ * @data: Data to pass to the function.
+ *
+ * Schedules the function @func for execution on the vCPU @cpu asynchronously,
+ * while all other vCPUs are sleeping.
+ *
+ * Unlike run_on_cpu and async_run_on_cpu, the function is run outside the
+ * BQL.
+ */
+void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data);
+
 /**
  * qemu_get_cpu:
  * @index: The CPUState@cpu_index value of the CPU to obtain.