Re: [PATCH 1/4] Add a mutex to guarantee single writer to qemu_logfile handle.

[Robert Foley]

On Thu, 7 Nov 2019 at 11:53, Alex Bennée <address@hidden> wrote:
>
> It wouldn't be the worst thing in the world to expose:
>
>   qemu_logfile_init()
>
> and make vl.c and main.c call it before the setup. Then you can drop the
> flag or even just g_assert(qemu_log_mutex_initialised) in qemu_set_log
> and qemu_set_logfile.
>
> In fact you could just use:
>
>   static void __attribute__((__constructor__)) qemu_logfile_init(void)
>
> and make the compiler do it for you.

All good ideas.  Will make the changes.
I agree, it is much cleaner to call init this way (constructor).  We
can assert that qemu_log_mutex.initialized on use of the mutex
(qemu_set_log and qemu_set_logfile).  Taking that one step further, we
could add simple helper functions for
qemu_logfile_mutex_lock()/unlock(), which g_assert() on
mutex.initialized first before lock/unlock.

Thanks,
-Rob
On Thu, 7 Nov 2019 at 11:53, Alex Bennée <address@hidden> wrote:
>
>
> Robert Foley <address@hidden> writes:
>
> > This is being added in preparation for using RCU with the logfile handle.
> > Also added qemu_logfile_init() for initializing the logfile mutex.
> >
> > Signed-off-by: Robert Foley <address@hidden>
> > ---
> >  util/log.c | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> >
> > diff --git a/util/log.c b/util/log.c
> > index 1ca13059ee..dff2f98c8c 100644
> > --- a/util/log.c
> > +++ b/util/log.c
> > @@ -24,8 +24,11 @@
> >  #include "qapi/error.h"
> >  #include "qemu/cutils.h"
> >  #include "trace/control.h"
> > +#include "qemu/thread.h"
> >
> >  static char *logfilename;
> > +static bool qemu_logfile_initialized;
> > +static QemuMutex qemu_logfile_mutex;
> >  FILE *qemu_logfile;
> >  int qemu_loglevel;
> >  static int log_append = 0;
> > @@ -49,6 +52,14 @@ int qemu_log(const char *fmt, ...)
> >      return ret;
> >  }
> >
> > +static void qemu_logfile_init(void)
> > +{
> > +    if (!qemu_logfile_initialized) {
> > +        qemu_mutex_init(&qemu_logfile_mutex);
> > +        qemu_logfile_initialized = true;
> > +    }
> > +}
> > +
> >  static bool log_uses_own_buffers;
> >
> >  /* enable or disable low levels log */
> > @@ -58,6 +69,12 @@ void qemu_set_log(int log_flags)
> >  #ifdef CONFIG_TRACE_LOG
> >      qemu_loglevel |= LOG_TRACE;
> >  #endif
> > +
> > +    /* Is there a better place to call this to init the logfile subsystem? 
> > */
> > +    if (!qemu_logfile_initialized) {
> > +        qemu_logfile_init();
> > +    }
>
> It wouldn't be the worst thing in the world to expose:
>
>   qemu_logfile_init()
>
> and make vl.c and main.c call it before the setup. Then you can drop the
> flag or even just g_assert(qemu_log_mutex_initialised) in qemu_set_log
> and qemu_set_logfile.
>
> In fact you could just use:
>
>   static void __attribute__((__constructor__)) qemu_logfile_init(void)
>
> and make the compiler do it for you.
>
> > +    qemu_mutex_lock(&qemu_logfile_mutex);
> >      if (!qemu_logfile &&
> >          (is_daemonized() ? logfilename != NULL : qemu_loglevel)) {
> >          if (logfilename) {
> > @@ -93,6 +110,7 @@ void qemu_set_log(int log_flags)
> >              log_append = 1;
> >          }
> >      }
> > +    qemu_mutex_unlock(&qemu_logfile_mutex);
> >      if (qemu_logfile &&
> >          (is_daemonized() ? logfilename == NULL : !qemu_loglevel)) {
> >          qemu_log_close();
> > @@ -114,6 +132,11 @@ void qemu_set_log_filename(const char *filename, Error 
> > **errp)
> >      char *pidstr;
> >      g_free(logfilename);
> >
> > +    /* Is there a better place to call this to init the logfile subsystem? 
> > */
> > +    if (!qemu_logfile_initialized) {
> > +        qemu_logfile_init();
> > +    }
> > +
> >      pidstr = strstr(filename, "%");
> >      if (pidstr) {
> >          /* We only accept one %d, no other format strings */
>
>
> --
> Alex Bennée