Re: [PATCH v2 3/6] Add a mutex to guarantee single writer to qemu_logfile handle.

["Alex Bennée" <alex.bennee@linaro.org>]

Robert Foley <robert.foley@linaro.org> writes:

> Also added qemu_logfile_init() for initializing the logfile mutex.
>
> Signed-off-by: Robert Foley <robert.foley@linaro.org>
> ---
> v2
>     - In qemu_set_log() moved location of mutex lock/unlock
>       due to cleanup changes.
> ---
> v1
>     - changed qemu_logfile_init() to use __constructor__.
> ---
>  util/log.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/util/log.c b/util/log.c
> index 417d16ec66..91ebb5c924 100644
> --- a/util/log.c
> +++ b/util/log.c
> @@ -24,8 +24,10 @@
>  #include "qapi/error.h"
>  #include "qemu/cutils.h"
>  #include "trace/control.h"
> +#include "qemu/thread.h"
>
>  static char *logfilename;
> +static QemuMutex qemu_logfile_mutex;
>  FILE *qemu_logfile;
>  int qemu_loglevel;
>  static int log_append = 0;
> @@ -49,6 +51,11 @@ int qemu_log(const char *fmt, ...)
>      return ret;
>  }
>
> +static void __attribute__((__constructor__)) qemu_logfile_init(void)
> +{
> +    qemu_mutex_init(&qemu_logfile_mutex);
> +}
> +
>  static bool log_uses_own_buffers;
>
>  /* enable or disable low levels log */
> @@ -70,7 +77,10 @@ void qemu_set_log(int log_flags)
>      if (qemu_loglevel && (!is_daemonized() || logfilename)) {
>          need_to_open_file = true;
>      }
> +    g_assert(qemu_logfile_mutex.initialized);
> +    qemu_mutex_lock(&qemu_logfile_mutex);
>      if (qemu_logfile && !need_to_open_file) {
> +        qemu_mutex_unlock(&qemu_logfile_mutex);
>          qemu_log_close();
>      } else if (!qemu_logfile && need_to_open_file) {
>          if (logfilename) {
> @@ -105,6 +115,7 @@ void qemu_set_log(int log_flags)
>  #endif
>              log_append = 1;
>          }
> +        qemu_mutex_unlock(&qemu_logfile_mutex);
>      }
>  }

This looks a bit odd. I can see it's fixed up in a later patch but I
guess the reason is to avoid a double lock when we get to
qemu_log_close(). In the cases of unavoidable temporary ugliness in a
patch series it is best to note the problem and mention it will be
cleaned up by a later patch in the series.

With an extra comment in the commit message:

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

>
> @@ -240,12 +251,15 @@ void qemu_log_flush(void)
>  /* Close the log file */
>  void qemu_log_close(void)
>  {
> +    g_assert(qemu_logfile_mutex.initialized);
> +    qemu_mutex_lock(&qemu_logfile_mutex);
>      if (qemu_logfile) {
>          if (qemu_logfile != stderr) {
>              fclose(qemu_logfile);
>          }
>          qemu_logfile = NULL;
>      }
> +    qemu_mutex_unlock(&qemu_logfile_mutex);
>  }
>
>  const QEMULogItem qemu_log_items[] = {


--
Alex Bennée