Re: [Qemu-devel] [PATCH 2/4] timer: protect timers_state's clock with seqlock

[Stefan Hajnoczi]

On Mon, Aug 05, 2013 at 03:33:24PM +0800, Liu Ping Fan wrote:
> diff --git a/cpus.c b/cpus.c
> index 85e743d..ab92db9 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -107,12 +107,17 @@ static int64_t qemu_icount;
>  typedef struct TimersState {
>      int64_t cpu_ticks_prev;
>      int64_t cpu_ticks_offset;
> +    /* QemuClock will be read out of BQL, so protect is with private lock.
> +     * As for cpu_ticks, no requirement to read it outside BQL.
> +     * Lock rule: innermost
> +     */

Please document exactly which fields the lock protects.

>  /* enable cpu_get_ticks() */
>  void cpu_enable_ticks(void)
>  {
> +    /* Here, the really thing protected by seqlock is cpu_clock. */

What is cpu_clock?

> @@ -364,6 +383,9 @@ static const VMStateDescription vmstate_timers = {
>  
>  void configure_icount(const char *option)
>  {
> +    QemuMutex *mutex = g_malloc0(sizeof(QemuMutex));
> +    qemu_mutex_init(mutex);
> +    seqlock_init(&timers_state.clock_seqlock, mutex);

We always set up this mutex, so it could be a field in timers_state.
That avoids the g_malloc() without g_free().