Re: [Qemu-devel] [PATCH 13/16] cpus-common: simplify locking for start_exclusive/end_exclusive

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

Paolo Bonzini <pbonzini@redhat.com> writes:

> It is not necessary to hold qemu_cpu_list_mutex throughout the
> exclusive section, because no other exclusive section can run
> while pending_cpus != 0.
>
> exclusive_idle() is called in cpu_exec_start(), and that prevents
> any CPUs created after start_exclusive() from entering cpu_exec()
> during an exclusive section.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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

> ---
>  cpus-common.c              | 11 ++++++++---
>  docs/tcg-exclusive.promela |  4 +++-
>  include/qom/cpu.h          |  4 ----
>  3 files changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/cpus-common.c b/cpus-common.c
> index 80aaf9b..429652c 100644
> --- a/cpus-common.c
> +++ b/cpus-common.c
> @@ -171,8 +171,7 @@ static inline void exclusive_idle(void)
>  }
>
>  /* Start an exclusive operation.
> -   Must only be called from outside cpu_exec, takes
> -   qemu_cpu_list_lock.   */
> +   Must only be called from outside cpu_exec.  */
>  void start_exclusive(void)
>  {
>      CPUState *other_cpu;
> @@ -191,11 +190,17 @@ void start_exclusive(void)
>      while (pending_cpus > 1) {
>          qemu_cond_wait(&exclusive_cond, &qemu_cpu_list_lock);
>      }
> +
> +    /* Can release mutex, no one will enter another exclusive
> +     * section until end_exclusive resets pending_cpus to 0.
> +     */
> +    qemu_mutex_unlock(&qemu_cpu_list_lock);
>  }
>
> -/* Finish an exclusive operation.  Releases qemu_cpu_list_lock.  */
> +/* Finish an exclusive operation.  */
>  void end_exclusive(void)
>  {
> +    qemu_mutex_lock(&qemu_cpu_list_lock);
>      pending_cpus = 0;
>      qemu_cond_broadcast(&exclusive_resume);
>      qemu_mutex_unlock(&qemu_cpu_list_lock);
> diff --git a/docs/tcg-exclusive.promela b/docs/tcg-exclusive.promela
> index 9e7d9e3..a8896e5 100644
> --- a/docs/tcg-exclusive.promela
> +++ b/docs/tcg-exclusive.promela
> @@ -97,9 +97,11 @@ byte has_waiter[N_CPUS];
>      do                                                            \
>        :: pending_cpus > 1 -> COND_WAIT(exclusive_cond, mutex);    \
>        :: else             -> break;                               \
> -    od
> +    od;                                                           \
> +    MUTEX_UNLOCK(mutex);
>
>  #define end_exclusive()                                           \
> +    MUTEX_LOCK(mutex);                                            \
>      pending_cpus = 0;                                             \
>      COND_BROADCAST(exclusive_resume);                             \
>      MUTEX_UNLOCK(mutex);
> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index f872614..934c07a 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -846,9 +846,6 @@ void cpu_exec_end(CPUState *cpu);
>   * cpu_exec are exited immediately.  CPUs that call cpu_exec_start
>   * during the exclusive section go to sleep until this CPU calls
>   * end_exclusive.
> - *
> - * Returns with the CPU list lock taken (which nests outside all
> - * other locks except the BQL).
>   */
>  void start_exclusive(void);
>
> @@ -856,7 +853,6 @@ void start_exclusive(void);
>   * end_exclusive:
>   *
>   * Concludes an exclusive execution section started by start_exclusive.
> - * Releases the CPU list lock.
>   */
>  void end_exclusive(void);


--
Alex Bennée