Re: [Qemu-devel] [PATCH v4 2/4] monitor: protect mon->fds with mon_lock

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> mon->fds were protected by BQL.  Now protect it by mon_lock so that it
> can even be used in monitor iothread.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  monitor.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/monitor.c b/monitor.c
> index 48882d28ae..9f361ec21e 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -213,7 +213,6 @@ struct Monitor {
>      BlockCompletionFunc *password_completion_cb;
>      void *password_opaque;
>      mon_cmd_t *cmd_table;
> -    QLIST_HEAD(,mon_fd_t) fds;
>      QTAILQ_ENTRY(Monitor) entry;
>  
>      /*
> @@ -225,6 +224,7 @@ struct Monitor {
>      /*
>       * Fields that are protected by the per-monitor lock.
>       */
> +    QLIST_HEAD(, mon_fd_t) fds;
>      QString *outbuf;
>      guint out_watch;
>      /* Read under either BQL or mon_lock, written with BQL+mon_lock.  */
> @@ -2207,6 +2207,7 @@ void qmp_getfd(const char *fdname, Error **errp)
>          return;
>      }
>  
> +    qemu_mutex_lock(&cur_mon->mon_lock);
>      QLIST_FOREACH(monfd, &cur_mon->fds, next) {
>          if (strcmp(monfd->name, fdname) != 0) {
>              continue;
> @@ -2214,6 +2215,7 @@ void qmp_getfd(const char *fdname, Error **errp)
>  
>          close(monfd->fd);
>          monfd->fd = fd;
> +        qemu_mutex_unlock(&cur_mon->mon_lock);

Why is it safe to have a close() in a mon_lock'd region?
We've got to make sure everything that happens in it is non-blocking.

Dave

>          return;
>      }
>  
> @@ -2222,12 +2224,14 @@ void qmp_getfd(const char *fdname, Error **errp)
>      monfd->fd = fd;
>  
>      QLIST_INSERT_HEAD(&cur_mon->fds, monfd, next);
> +    qemu_mutex_unlock(&cur_mon->mon_lock);
>  }
>  
>  void qmp_closefd(const char *fdname, Error **errp)
>  {
>      mon_fd_t *monfd;
>  
> +    qemu_mutex_lock(&cur_mon->mon_lock);
>      QLIST_FOREACH(monfd, &cur_mon->fds, next) {
>          if (strcmp(monfd->name, fdname) != 0) {
>              continue;
> @@ -2237,9 +2241,11 @@ void qmp_closefd(const char *fdname, Error **errp)
>          close(monfd->fd);
>          g_free(monfd->name);
>          g_free(monfd);
> +        qemu_mutex_unlock(&cur_mon->mon_lock);
>          return;
>      }
>  
> +    qemu_mutex_unlock(&cur_mon->mon_lock);
>      error_setg(errp, QERR_FD_NOT_FOUND, fdname);
>  }
>  
> @@ -2247,6 +2253,7 @@ int monitor_get_fd(Monitor *mon, const char *fdname, Error **errp)
>  {
>      mon_fd_t *monfd;
>  
> +    qemu_mutex_lock(&mon->mon_lock);
>      QLIST_FOREACH(monfd, &mon->fds, next) {
>          int fd;
>  
> @@ -2260,10 +2267,12 @@ int monitor_get_fd(Monitor *mon, const char *fdname, Error **errp)
>          QLIST_REMOVE(monfd, next);
>          g_free(monfd->name);
>          g_free(monfd);
> +        qemu_mutex_unlock(&mon->mon_lock);
>  
>          return fd;
>      }
>  
> +    qemu_mutex_unlock(&mon->mon_lock);
>      error_setg(errp, "File descriptor named '%s' has not been found", fdname);
>      return -1;
>  }
> -- 
> 2.14.3
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK