Re: [PATCH v6 06/12] monitor: Make current monitor a per-coroutine property

[Markus Armbruster <armbru@redhat.com>]

Kevin Wolf <kwolf@redhat.com> writes:

> This way, a monitor command handler will still be able to access the
> current monitor, but when it yields, all other code code will correctly
> get NULL from monitor_cur().
>
> Outside of coroutine context, qemu_coroutine_self() returns the leader
> coroutine of the current thread.

Unsaid: you use it as a hash table key to map from coroutine to monitor,
and for that you need it to return a value unique to the coroutine in
coroutine context, and a value unique to the thread outside coroutine
context.  Which qemu_coroutine_self() does.  Correct?

The hash table works, but I hate it just as much as I hate
pthread_getspecific() / pthread_setspecific().

What we have here is a need for coroutine-local data.  Feels like a
perfectly natural concept to me.

Are we going to create another hash table whenever we need another piece
of coroutine-local data?  Or shall we reuse the hash table, suitably
renamed and moved to another file?

Why not simply associate an opaque pointer with each coroutine?  All it
takes is one more member of struct Coroutine.  Whatever creates the
coroutine decides what to use it for.  The monitor coroutine would use
it to point to the monitor.

At least, discuss the design alternatives in the commit message.

> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  include/monitor/monitor.h |  2 +-
>  monitor/hmp.c             |  4 ++--
>  monitor/monitor.c         | 27 +++++++++++++++++++++------
>  qapi/qmp-dispatch.c       |  4 ++--
>  stubs/monitor-core.c      |  2 +-
>  5 files changed, 27 insertions(+), 12 deletions(-)
>
> diff --git a/include/monitor/monitor.h b/include/monitor/monitor.h
> index 43cc746078..16072e325c 100644
> --- a/include/monitor/monitor.h
> +++ b/include/monitor/monitor.h
> @@ -13,7 +13,7 @@ typedef struct MonitorOptions MonitorOptions;
>  extern QemuOptsList qemu_mon_opts;
>  
>  Monitor *monitor_cur(void);
> -void monitor_set_cur(Monitor *mon);
> +void monitor_set_cur(Coroutine *co, Monitor *mon);
>  bool monitor_cur_is_qmp(void);
>  
>  void monitor_init_globals(void);
> diff --git a/monitor/hmp.c b/monitor/hmp.c
> index 79be6f26de..3e73a4c3ce 100644
> --- a/monitor/hmp.c
> +++ b/monitor/hmp.c
> @@ -1082,9 +1082,9 @@ void handle_hmp_command(MonitorHMP *mon, const char *cmdline)
>  
>      /* old_mon is non-NULL when called from qmp_human_monitor_command() */
>      old_mon = monitor_cur();
> -    monitor_set_cur(&mon->common);
> +    monitor_set_cur(qemu_coroutine_self(), &mon->common);
>      cmd->cmd(&mon->common, qdict);
> -    monitor_set_cur(old_mon);
> +    monitor_set_cur(qemu_coroutine_self(), old_mon);
>  
>      qobject_unref(qdict);
>  }
> diff --git a/monitor/monitor.c b/monitor/monitor.c
> index 182ba136b4..35003bb486 100644
> --- a/monitor/monitor.c
> +++ b/monitor/monitor.c
> @@ -58,24 +58,38 @@ IOThread *mon_iothread;
>  /* Bottom half to dispatch the requests received from I/O thread */
>  QEMUBH *qmp_dispatcher_bh;
>  
> -/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
> +/*
> + * Protects mon_list, monitor_qapi_event_state, coroutine_mon,
> + * monitor_destroyed.
> + */
>  QemuMutex monitor_lock;
>  static GHashTable *monitor_qapi_event_state;
> +static GHashTable *coroutine_mon; /* Maps Coroutine* to Monitor* */
>  
>  MonitorList mon_list;
>  int mon_refcount;
>  static bool monitor_destroyed;
>  
> -static __thread Monitor *cur_monitor;
> -
>  Monitor *monitor_cur(void)
>  {
> -    return cur_monitor;
> +    Monitor *mon;
> +
> +    qemu_mutex_lock(&monitor_lock);
> +    mon = g_hash_table_lookup(coroutine_mon, qemu_coroutine_self());
> +    qemu_mutex_unlock(&monitor_lock);
> +
> +    return mon;
>  }
>  
> -void monitor_set_cur(Monitor *mon)
> +void monitor_set_cur(Coroutine *co, Monitor *mon)
>  {
> -    cur_monitor = mon;
> +    qemu_mutex_lock(&monitor_lock);
> +    if (mon) {
> +        g_hash_table_replace(coroutine_mon, co, mon);
> +    } else {
> +        g_hash_table_remove(coroutine_mon, co);
> +    }
> +    qemu_mutex_unlock(&monitor_lock);
>  }

You really need a contract now: any call to monitor_set_cur() with a
non-null @mon must be followed by a call with a null @mon.

>  
>  /**
> @@ -613,6 +627,7 @@ void monitor_init_globals_core(void)
>  {
>      monitor_qapi_event_init();
>      qemu_mutex_init(&monitor_lock);
> +    coroutine_mon = g_hash_table_new(NULL, NULL);
>  
>      /*
>       * The dispatcher BH must run in the main loop thread, since we
> diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c
> index 2fdbc0fba4..5677ba92ca 100644
> --- a/qapi/qmp-dispatch.c
> +++ b/qapi/qmp-dispatch.c
> @@ -154,11 +154,11 @@ QDict *qmp_dispatch(const QmpCommandList *cmds, QObject *request,
>      }
>  
>      assert(monitor_cur() == NULL);
> -    monitor_set_cur(cur_mon);
> +    monitor_set_cur(qemu_coroutine_self(), cur_mon);
>  
>      cmd->fn(args, &ret, &err);
>  
> -    monitor_set_cur(NULL);
> +    monitor_set_cur(qemu_coroutine_self(), NULL);
>      qobject_unref(args);
>      if (err) {
>          /* or assert(!ret) after reviewing all handlers: */
> diff --git a/stubs/monitor-core.c b/stubs/monitor-core.c
> index e493df1027..635e37a6ba 100644
> --- a/stubs/monitor-core.c
> +++ b/stubs/monitor-core.c
> @@ -8,7 +8,7 @@ Monitor *monitor_cur(void)
>      return NULL;
>  }
>  
> -void monitor_set_cur(Monitor *mon)
> +void monitor_set_cur(Coroutine *co, Monitor *mon)
>  {
>  }