[Qemu-devel] [PATCH v2] qemu-char: eliminate busy waiting on can_read returning zero

[Paolo Bonzini <pbonzini@redhat.com>]

The character backend refactoring introduced an undesirable busy wait.
The busy wait happens if can_read returns zero and there is data available
on the character device's file descriptor.  Then, the I/O watch will
fire continuously and, with TCG, the CPU thread will never run.

    1) Char backend asks front end if it can write
    2) Front end says no
    3) poll() finds the char backend's descriptor is available
    4) Goto (1)

What we really want is this (note that step 3 avoids the busy wait):

    1) Char backend asks front end if it can write
    2) Front end says no
    3) poll() goes on without char backend's descriptor
    4) Goto (1) until qemu_chr_accept_input() called

    5) Char backend asks front end if it can write
    6) Front end says yes
    7) poll() finds the char backend's descriptor is available
    8) Backend handler called

After this patch, the IOWatchPoll source and the watch source are
separated.  The IOWatchPoll is simply a hook that runs during the prepare
phase on each main loop iteration.  The hook adds/removes the actual
source depending on the return value from can_read.

A simple reproducer is

    qemu-system-i386 -serial mon:stdio

... followed by banging on the terminal as much as you can. :)  Without
this patch, emulation will hang.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
        v1->v2: use g_source_get_context to find if the watch was active

 qemu-char.c | 62 +++++++++++++++++++------------------------------------------
 1 file changed, 19 insertions(+), 43 deletions(-)

diff --git a/qemu-char.c b/qemu-char.c
index e5eb8dd..85ebcf9 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -594,65 +594,51 @@ int recv_all(int fd, void *_buf, int len1, bool single_read)
 
 typedef struct IOWatchPoll
 {
+    GSource parent;
+
     GSource *src;
-    int max_size;
 
     IOCanReadHandler *fd_can_read;
     void *opaque;
-
-    QTAILQ_ENTRY(IOWatchPoll) node;
 } IOWatchPoll;
 
-static QTAILQ_HEAD(, IOWatchPoll) io_watch_poll_list =
-    QTAILQ_HEAD_INITIALIZER(io_watch_poll_list);
-
 static IOWatchPoll *io_watch_poll_from_source(GSource *source)
 {
-    IOWatchPoll *i;
-
-    QTAILQ_FOREACH(i, &io_watch_poll_list, node) {
-        if (i->src == source) {
-            return i;
-        }
-    }
-
-    return NULL;
+    return container_of(source, IOWatchPoll, parent);
 }
 
 static gboolean io_watch_poll_prepare(GSource *source, gint *timeout_)
 {
     IOWatchPoll *iwp = io_watch_poll_from_source(source);
-
-    iwp->max_size = iwp->fd_can_read(iwp->opaque);
-    if (iwp->max_size == 0) {
+    bool now_active = iwp->fd_can_read(iwp->opaque) > 0;
+    bool was_active = g_source_get_context(iwp->src) != NULL;
+    if (was_active == now_active) {
         return FALSE;
     }
 
-    return g_io_watch_funcs.prepare(source, timeout_);
+    if (now_active) {
+        g_source_attach(iwp->src, NULL);
+    } else {
+        g_source_remove(g_source_get_id(iwp->src));
+    }
+    return FALSE;
 }
 
 static gboolean io_watch_poll_check(GSource *source)
 {
-    IOWatchPoll *iwp = io_watch_poll_from_source(source);
-
-    if (iwp->max_size == 0) {
-        return FALSE;
-    }
-
-    return g_io_watch_funcs.check(source);
+    return FALSE;
 }
 
 static gboolean io_watch_poll_dispatch(GSource *source, GSourceFunc callback,
                                        gpointer user_data)
 {
-    return g_io_watch_funcs.dispatch(source, callback, user_data);
+    abort();
 }
 
 static void io_watch_poll_finalize(GSource *source)
 {
     IOWatchPoll *iwp = io_watch_poll_from_source(source);
-    QTAILQ_REMOVE(&io_watch_poll_list, iwp, node);
-    g_io_watch_funcs.finalize(source);
+    g_source_unref(iwp->src);
 }
 
 static GSourceFuncs io_watch_poll_funcs = {
@@ -669,24 +655,14 @@ static guint io_add_watch_poll(GIOChannel *channel,
                                gpointer user_data)
 {
     IOWatchPoll *iwp;
-    GSource *src;
-    guint tag;
-
-    src = g_io_create_watch(channel, G_IO_IN | G_IO_ERR | G_IO_HUP);
-    g_source_set_funcs(src, &io_watch_poll_funcs);
-    g_source_set_callback(src, (GSourceFunc)fd_read, user_data, NULL);
-    tag = g_source_attach(src, NULL);
-    g_source_unref(src);
 
-    iwp = g_malloc0(sizeof(*iwp));
-    iwp->src = src;
-    iwp->max_size = 0;
+    iwp = (IOWatchPoll *) g_source_new(&io_watch_poll_funcs, sizeof(IOWatchPoll));
     iwp->fd_can_read = fd_can_read;
     iwp->opaque = user_data;
+    iwp->src = g_io_create_watch(channel, G_IO_IN | G_IO_ERR | G_IO_HUP);
+    g_source_set_callback(iwp->src, (GSourceFunc)fd_read, user_data, NULL);
 
-    QTAILQ_INSERT_HEAD(&io_watch_poll_list, iwp, node);
-
-    return tag;
+    return g_source_attach(&iwp->parent, NULL);
 }
 
 #ifndef _WIN32
-- 
1.8.2