monitor: do not use mb_read/mb_set for suspend_cnt

Clean up monitor_event to just use monitor_suspend/monitor_resume,
using mon->mux_out to protect against incorrect nesting (especially
on startup).

The only remaining case of reading suspend_cnt is in the can_read
callback, which is just advisory and can use qatomic_read.

As an extra benefit, mux_out is now simply protected by mon_lock.
Also, moving the prompt to the beginning of the main loop removes
it from the output in some error cases where QEMU does not actually
start successfully.  It is not a full fix and it would be nice to
also remove the monitor heading, but this is already a small (though
unintentional) improvement.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

3 files changed, 24 insertions, 23 deletions

diff --git a/monitor/hmp.c b/monitor/hmp.c
index 5cab56d..69c1b7e 100644
--- a/monitor/hmp.c
+++ b/monitor/hmp.c
@@ -1401,45 +1401,42 @@ static void monitor_read(void *opaque, const uint8_t *buf, int size)
 static void monitor_event(void *opaque, QEMUChrEvent event)
 {
     Monitor *mon = opaque;
-    MonitorHMP *hmp_mon = container_of(mon, MonitorHMP, common);
 
     switch (event) {
     case CHR_EVENT_MUX_IN:
         qemu_mutex_lock(&mon->mon_lock);
-        mon->mux_out = 0;
-        qemu_mutex_unlock(&mon->mon_lock);
-        if (mon->reset_seen) {
-            readline_restart(hmp_mon->rs);
+        if (mon->mux_out) {
+            mon->mux_out = 0;
             monitor_resume(mon);
-            monitor_flush(mon);
-        } else {
-            qatomic_mb_set(&mon->suspend_cnt, 0);
         }
+        qemu_mutex_unlock(&mon->mon_lock);
         break;
 
     case CHR_EVENT_MUX_OUT:
-        if (mon->reset_seen) {
-            if (qatomic_mb_read(&mon->suspend_cnt) == 0) {
-                monitor_printf(mon, "\n");
+        qemu_mutex_lock(&mon->mon_lock);
+        if (!mon->mux_out) {
+            if (mon->reset_seen && !mon->suspend_cnt) {
+                monitor_puts_locked(mon, "\n");
+            } else {
+                monitor_flush_locked(mon);
             }
-            monitor_flush(mon);
             monitor_suspend(mon);
-        } else {
-            qatomic_inc(&mon->suspend_cnt);
+            mon->mux_out = 1;
         }
-        qemu_mutex_lock(&mon->mon_lock);
-        mon->mux_out = 1;
         qemu_mutex_unlock(&mon->mon_lock);
         break;
 
     case CHR_EVENT_OPENED:
         monitor_printf(mon, "QEMU %s monitor - type 'help' for more "
                        "information\n", QEMU_VERSION);
+        qemu_mutex_lock(&mon->mon_lock);
+        mon->reset_seen = 1;
         if (!mon->mux_out) {
-            readline_restart(hmp_mon->rs);
-            readline_show_prompt(hmp_mon->rs);
+            /* Suspend-resume forces the prompt to be printed.  */
+            monitor_suspend(mon);
+            monitor_resume(mon);
         }
-        mon->reset_seen = 1;
+        qemu_mutex_unlock(&mon->mon_lock);
         mon_refcount++;
         break;
 
diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
index 53e3808..61c9b69 100644
--- a/monitor/monitor-internal.h
+++ b/monitor/monitor-internal.h
@@ -94,7 +94,6 @@ typedef struct HMPCommand {
 
 struct Monitor {
     CharBackend chr;
-    int reset_seen;
     int suspend_cnt;            /* Needs to be accessed atomically */
     bool is_qmp;
     bool skip_flush;
@@ -115,8 +114,8 @@ struct Monitor {
     QLIST_HEAD(, mon_fd_t) fds;
     GString *outbuf;
     guint out_watch;
-    /* Read under either BQL or mon_lock, written with BQL+mon_lock.  */
     int mux_out;
+    int reset_seen;
 };
 
 struct MonitorHMP {
diff --git a/monitor/monitor.c b/monitor/monitor.c
index 20e33e2..15f9753 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -569,10 +569,15 @@ static void monitor_accept_input(void *opaque)
 {
     Monitor *mon = opaque;
 
-    if (!monitor_is_qmp(mon)) {
+    qemu_mutex_lock(&mon->mon_lock);
+    if (!monitor_is_qmp(mon) && mon->reset_seen) {
         MonitorHMP *hmp_mon = container_of(mon, MonitorHMP, common);
         assert(hmp_mon->rs);
+        readline_restart(hmp_mon->rs);
+        qemu_mutex_unlock(&mon->mon_lock);
         readline_show_prompt(hmp_mon->rs);
+    } else {
+        qemu_mutex_unlock(&mon->mon_lock);
     }
 
     qemu_chr_fe_accept_input(&mon->chr);
@@ -603,7 +608,7 @@ int monitor_can_read(void *opaque)
 {
     Monitor *mon = opaque;
 
-    return !qatomic_mb_read(&mon->suspend_cnt);
+    return !qatomic_read(&mon->suspend_cnt);
 }
 
 void monitor_list_append(Monitor *mon)