Add a monitor_destroyed global to check if monitor_cleanup() has been
already called. In this case, don't insert the new monitor in the
list, but free it instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 monitor.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/monitor.c b/monitor.c
index fffeb27ef9..7fe89daa87 100644
--- a/monitor.c
+++ b/monitor.c
@@ -263,10 +263,11 @@ typedef struct QMPRequest QMPRequest;
 /* QMP checker flags */
 #define QMP_ACCEPT_UNKNOWNS 1
 
-/* Protects mon_list, monitor_qapi_event_state.  */
+/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
 static QemuMutex monitor_lock;
 static GHashTable *monitor_qapi_event_state;
 static QTAILQ_HEAD(mon_list, Monitor) mon_list;
+static bool monitor_destroyed;
 
 /* Protects mon_fdsets */
 static QemuMutex mon_fdsets_lock;
@@ -4536,8 +4537,16 @@ void error_vprintf_unless_qmp(const char *fmt, va_list ap)
 static void monitor_list_append(Monitor *mon)
 {
     qemu_mutex_lock(&monitor_lock);
-    QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
+    if (!monitor_destroyed) {
+        QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
+        mon = NULL;
+    }
     qemu_mutex_unlock(&monitor_lock);
+
+    if (mon) {
+        monitor_data_destroy(mon);
+        g_free(mon);
+    }
 }
 
 static void monitor_qmp_setup_handlers_bh(void *opaque)
@@ -4631,6 +4640,7 @@ void monitor_cleanup(void)
 
     /* Flush output buffers and destroy monitors */
     qemu_mutex_lock(&monitor_lock);
+    monitor_destroyed = true;
     QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
         QTAILQ_REMOVE(&mon_list, mon, entry);
         monitor_flush(mon);
-- 
2.19.0.271.gfe8321ec05

Marc-André Lureau <marcandre.lureau@redhat.com> writes:

> Add a monitor_destroyed global to check if monitor_cleanup() has been
> already called. In this case, don't insert the new monitor in the
> list, but free it instead.
>
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

The commit message explains what the patch does, but not why we want to
do it.

> ---
>  monitor.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/monitor.c b/monitor.c
> index fffeb27ef9..7fe89daa87 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -263,10 +263,11 @@ typedef struct QMPRequest QMPRequest;
>  /* QMP checker flags */
>  #define QMP_ACCEPT_UNKNOWNS 1
>  
> -/* Protects mon_list, monitor_qapi_event_state.  */
> +/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
>  static QemuMutex monitor_lock;
>  static GHashTable *monitor_qapi_event_state;
>  static QTAILQ_HEAD(mon_list, Monitor) mon_list;
> +static bool monitor_destroyed;
>  
>  /* Protects mon_fdsets */
>  static QemuMutex mon_fdsets_lock;
> @@ -4536,8 +4537,16 @@ void error_vprintf_unless_qmp(const char *fmt, va_list ap)
>  static void monitor_list_append(Monitor *mon)
>  {
>      qemu_mutex_lock(&monitor_lock);
> -    QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
> +    if (!monitor_destroyed) {
> +        QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
> +        mon = NULL;
> +    }
>      qemu_mutex_unlock(&monitor_lock);
> +
> +    if (mon) {
> +        monitor_data_destroy(mon);
> +        g_free(mon);
> +    }
>  }
>  
>  static void monitor_qmp_setup_handlers_bh(void *opaque)
> @@ -4631,6 +4640,7 @@ void monitor_cleanup(void)
>  
>      /* Flush output buffers and destroy monitors */
>      qemu_mutex_lock(&monitor_lock);
> +    monitor_destroyed = true;
>      QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
>          QTAILQ_REMOVE(&mon_list, mon, entry);
>          monitor_flush(mon);

monitor_cleanup() is one of the last things main() calls before it
returns.  If another thread creates a monitor afterwards, it won't be
cleaned up.  I figure that's the reason we want this patch.  There might
be more serious issues than failure to clean up.  Please explain in your
commit message.

monitor_list_append() is called by monitor_init(), either directly right
before it returns, or via a bottom half if mon->use_io_thread.
Therefore, @monitor_destroyed can only be true if monitor_init()
commonly runs in a thread other than the main thread.  Please give an
example where it does, in your commit message.

Hi
On Mon, Dec 3, 2018 at 12:59 PM Markus Armbruster <armbru@redhat.com> wrote:
>
> Marc-André Lureau <marcandre.lureau@redhat.com> writes:
>
> > Add a monitor_destroyed global to check if monitor_cleanup() has been
> > already called. In this case, don't insert the new monitor in the
> > list, but free it instead.
> >
> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
>
> The commit message explains what the patch does, but not why we want to
> do it.
>
> > ---
> >  monitor.c | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/monitor.c b/monitor.c
> > index fffeb27ef9..7fe89daa87 100644
> > --- a/monitor.c
> > +++ b/monitor.c
> > @@ -263,10 +263,11 @@ typedef struct QMPRequest QMPRequest;
> >  /* QMP checker flags */
> >  #define QMP_ACCEPT_UNKNOWNS 1
> >
> > -/* Protects mon_list, monitor_qapi_event_state.  */
> > +/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
> >  static QemuMutex monitor_lock;
> >  static GHashTable *monitor_qapi_event_state;
> >  static QTAILQ_HEAD(mon_list, Monitor) mon_list;
> > +static bool monitor_destroyed;
> >
> >  /* Protects mon_fdsets */
> >  static QemuMutex mon_fdsets_lock;
> > @@ -4536,8 +4537,16 @@ void error_vprintf_unless_qmp(const char *fmt, va_list ap)
> >  static void monitor_list_append(Monitor *mon)
> >  {
> >      qemu_mutex_lock(&monitor_lock);
> > -    QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
> > +    if (!monitor_destroyed) {
> > +        QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
> > +        mon = NULL;
> > +    }
> >      qemu_mutex_unlock(&monitor_lock);
> > +
> > +    if (mon) {
> > +        monitor_data_destroy(mon);
> > +        g_free(mon);
> > +    }
> >  }
> >
> >  static void monitor_qmp_setup_handlers_bh(void *opaque)
> > @@ -4631,6 +4640,7 @@ void monitor_cleanup(void)
> >
> >      /* Flush output buffers and destroy monitors */
> >      qemu_mutex_lock(&monitor_lock);
> > +    monitor_destroyed = true;
> >      QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
> >          QTAILQ_REMOVE(&mon_list, mon, entry);
> >          monitor_flush(mon);
>
> monitor_cleanup() is one of the last things main() calls before it
> returns.  If another thread creates a monitor afterwards, it won't be
> cleaned up.  I figure that's the reason we want this patch.  There might
> be more serious issues than failure to clean up.  Please explain in your
> commit message.

Is this clearer?

    monitor_cleanup() is one of the last things main() calls before it
    returns.  In the following patch, monitor_cleanup() will release the
    monitor_lock during flushing. There may be pending commands to insert
    new monitors, which would modify the mon_list during iteration, and
    the clean-up could thus miss those new insertions.

    Add a monitor_destroyed global to check if monitor_cleanup() has been
    already called. In this case, don't insert the new monitor in the
    list, but free it instead.

> monitor_list_append() is called by monitor_init(), either directly right
> before it returns, or via a bottom half if mon->use_io_thread.
> Therefore, @monitor_destroyed can only be true if monitor_init()
> commonly runs in a thread other than the main thread.  Please give an
> example where it does, in your commit message.

Marc-André Lureau <marcandre.lureau@redhat.com> writes:

> Hi
> On Mon, Dec 3, 2018 at 12:59 PM Markus Armbruster <armbru@redhat.com> wrote:
>>
>> Marc-André Lureau <marcandre.lureau@redhat.com> writes:
>>
>> > Add a monitor_destroyed global to check if monitor_cleanup() has been
>> > already called. In this case, don't insert the new monitor in the
>> > list, but free it instead.
>> >
>> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
>>
>> The commit message explains what the patch does, but not why we want to
>> do it.
>>
>> > ---
>> >  monitor.c | 14 ++++++++++++--
>> >  1 file changed, 12 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/monitor.c b/monitor.c
>> > index fffeb27ef9..7fe89daa87 100644
>> > --- a/monitor.c
>> > +++ b/monitor.c
>> > @@ -263,10 +263,11 @@ typedef struct QMPRequest QMPRequest;
>> >  /* QMP checker flags */
>> >  #define QMP_ACCEPT_UNKNOWNS 1
>> >
>> > -/* Protects mon_list, monitor_qapi_event_state.  */
>> > +/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
>> >  static QemuMutex monitor_lock;
>> >  static GHashTable *monitor_qapi_event_state;
>> >  static QTAILQ_HEAD(mon_list, Monitor) mon_list;
>> > +static bool monitor_destroyed;
>> >
>> >  /* Protects mon_fdsets */
>> >  static QemuMutex mon_fdsets_lock;
>> > @@ -4536,8 +4537,16 @@ void error_vprintf_unless_qmp(const char *fmt, va_list ap)
>> >  static void monitor_list_append(Monitor *mon)
>> >  {
>> >      qemu_mutex_lock(&monitor_lock);
>> > -    QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
>> > +    if (!monitor_destroyed) {
>> > +        QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
>> > +        mon = NULL;
>> > +    }
>> >      qemu_mutex_unlock(&monitor_lock);
>> > +
>> > +    if (mon) {
>> > +        monitor_data_destroy(mon);
>> > +        g_free(mon);
>> > +    }
>> >  }
>> >
>> >  static void monitor_qmp_setup_handlers_bh(void *opaque)
>> > @@ -4631,6 +4640,7 @@ void monitor_cleanup(void)
>> >
>> >      /* Flush output buffers and destroy monitors */
>> >      qemu_mutex_lock(&monitor_lock);
>> > +    monitor_destroyed = true;
>> >      QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
>> >          QTAILQ_REMOVE(&mon_list, mon, entry);
>> >          monitor_flush(mon);
>>
>> monitor_cleanup() is one of the last things main() calls before it
>> returns.  If another thread creates a monitor afterwards, it won't be
>> cleaned up.  I figure that's the reason we want this patch.  There might
>> be more serious issues than failure to clean up.  Please explain in your
>> commit message.
>
> Is this clearer?
>
>     monitor_cleanup() is one of the last things main() calls before it
>     returns.  In the following patch, monitor_cleanup() will release the
>     monitor_lock during flushing. There may be pending commands to insert
>     new monitors, which would modify the mon_list during iteration, and
>     the clean-up could thus miss those new insertions.
>
>     Add a monitor_destroyed global to check if monitor_cleanup() has been
>     already called. In this case, don't insert the new monitor in the
>     list, but free it instead.

Yes.

The solution feels a bit clunky: it duplicates part of the work
monitor_cleanup() does into monitor_list_append().  Works because the
part it doesn't duplicate needn't be done in this case: removing from
@mon_list, and monitor_flush().

I suspect a cleaner solution would involve the main thread telling other
threads to terminate, waiting for their termination with pthread_join(),
and only then do final cleanup.

I'm not asking for that solution now.  A clunky fix we have is better
than a refactoring we don't have.  A comment admitting the clunkiness
would be nice.

>> monitor_list_append() is called by monitor_init(), either directly right
>> before it returns, or via a bottom half if mon->use_io_thread.
>> Therefore, @monitor_destroyed can only be true if monitor_init()
>> commonly runs in a thread other than the main thread.  Please give an
>> example where it does, in your commit message.

On Mon, Oct 29, 2018 at 04:57:32PM +0400, Marc-André Lureau wrote:
> Add a monitor_destroyed global to check if monitor_cleanup() has been
> already called. In this case, don't insert the new monitor in the
> list, but free it instead.

Pure question: how to trigger the condition when doing monitor
cleanups?

I can understand the problem to be fixed in the follow-up patch, but I
don't know whether it's that helpful to have this patch though,
especially considering that we're reaching softfreeze.

> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  monitor.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/monitor.c b/monitor.c
> index fffeb27ef9..7fe89daa87 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -263,10 +263,11 @@ typedef struct QMPRequest QMPRequest;
>  /* QMP checker flags */
>  #define QMP_ACCEPT_UNKNOWNS 1
>  
> -/* Protects mon_list, monitor_qapi_event_state.  */
> +/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
>  static QemuMutex monitor_lock;
>  static GHashTable *monitor_qapi_event_state;
>  static QTAILQ_HEAD(mon_list, Monitor) mon_list;
> +static bool monitor_destroyed;
>  
>  /* Protects mon_fdsets */
>  static QemuMutex mon_fdsets_lock;
> @@ -4536,8 +4537,16 @@ void error_vprintf_unless_qmp(const char *fmt, va_list ap)
>  static void monitor_list_append(Monitor *mon)
>  {
>      qemu_mutex_lock(&monitor_lock);
> -    QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
> +    if (!monitor_destroyed) {
> +        QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
> +        mon = NULL;
> +    }
>      qemu_mutex_unlock(&monitor_lock);
> +
> +    if (mon) {
> +        monitor_data_destroy(mon);
> +        g_free(mon);
> +    }
>  }
>  
>  static void monitor_qmp_setup_handlers_bh(void *opaque)
> @@ -4631,6 +4640,7 @@ void monitor_cleanup(void)
>  
>      /* Flush output buffers and destroy monitors */
>      qemu_mutex_lock(&monitor_lock);
> +    monitor_destroyed = true;
>      QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
>          QTAILQ_REMOVE(&mon_list, mon, entry);
>          monitor_flush(mon);
> -- 
> 2.19.0.271.gfe8321ec05
> 

Regards,

-- 
Peter Xu