Re: [Qemu-devel] [3/4] cpus: protect TimerState writes with a spinlock

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

Hi, Paolo!

Seems that this one breaks the record/replay.

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> In the next patch, we will need to write cpu_ticks_offset from any
> thread, even outside the BQL.  Currently, it is protected by the BQL
> just because cpu_enable_ticks and cpu_disable_ticks happen to hold it,
> but the critical sections are well delimited and it's easy to remove
> the BQL dependency.
> 
> Add a spinlock that matches vm_clock_seqlock, and hold it when writing
> to the TimerState.  This also lets us fix cpu_update_icount when 64-bit
> atomics are not available.
> 
> Fields of TiemrState are reordered to avoid padding.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  cpus.c | 72 ++++++++++++++++++++++++++++++++++++++--------------------
>  1 file changed, 47 insertions(+), 25 deletions(-)
> 
> diff --git a/cpus.c b/cpus.c
> index 680706aefa..63ddd4fd21 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -129,21 +129,27 @@ typedef struct TimersState {
>      int64_t cpu_ticks_prev;
>      int64_t cpu_ticks_offset;
> 
> -    /* cpu_clock_offset can be read out of BQL, so protect it with
> -     * this lock.
> +    /* Protect fields that can be respectively read outside the
> +     * BQL, and written from multiple threads.
>       */
>      QemuSeqLock vm_clock_seqlock;
> -    int64_t cpu_clock_offset;
> -    int32_t cpu_ticks_enabled;
> +    QemuSpin vm_clock_lock;
> +
> +    int16_t cpu_ticks_enabled;
> 
>      /* Conversion factor from emulated instructions to virtual clock ticks.  */
> -    int icount_time_shift;
> +    int16_t icount_time_shift;
> +
>      /* Compensate for varying guest execution speed.  */
>      int64_t qemu_icount_bias;
> +
> +    int64_t vm_clock_warp_start;
> +    int64_t cpu_clock_offset;
> +
>      /* Only written by TCG thread */
>      int64_t qemu_icount;
> +
>      /* for adjusting icount */
> -    int64_t vm_clock_warp_start;
>      QEMUTimer *icount_rt_timer;
>      QEMUTimer *icount_vm_timer;
>      QEMUTimer *icount_warp_timer;
> @@ -244,11 +250,15 @@ void cpu_update_icount(CPUState *cpu)
>      int64_t executed = cpu_get_icount_executed(cpu);
>      cpu->icount_budget -= executed;
> 
> -#ifdef CONFIG_ATOMIC64
> +#ifndef CONFIG_ATOMIC64
> +    seqlock_write_lock(&timers_state.vm_clock_seqlock,
> +                       &timers_state.vm_clock_lock);
> +#endif
>      atomic_set__nocheck(&timers_state.qemu_icount,
>                          timers_state.qemu_icount + executed);
> -#else /* FIXME: we need 64bit atomics to do this safely */
> -    timers_state.qemu_icount += executed;
> +#ifndef CONFIG_ATOMIC64
> +    seqlock_write_unlock(&timers_state.vm_clock_seqlock,
> +                         &timers_state.vm_clock_lock);
>  #endif
>  }
> 
> @@ -369,14 +379,15 @@ int64_t cpu_get_clock(void)
>   */
>  void cpu_enable_ticks(void)
>  {
> -    /* Here, the really thing protected by seqlock is cpu_clock_offset. */
> -    seqlock_write_begin(&timers_state.vm_clock_seqlock);
> +    seqlock_write_lock(&timers_state.vm_clock_seqlock,
> +                       &timers_state.vm_clock_lock);
>      if (!timers_state.cpu_ticks_enabled) {
>          timers_state.cpu_ticks_offset -= cpu_get_host_ticks();
>          timers_state.cpu_clock_offset -= get_clock();
>          timers_state.cpu_ticks_enabled = 1;
>      }
> -    seqlock_write_end(&timers_state.vm_clock_seqlock);
> +    seqlock_write_unlock(&timers_state.vm_clock_seqlock,
> +                       &timers_state.vm_clock_lock);
>  }
> 
>  /* disable cpu_get_ticks() : the clock is stopped. You must not call
> @@ -385,14 +396,15 @@ void cpu_enable_ticks(void)
>   */
>  void cpu_disable_ticks(void)
>  {
> -    /* Here, the really thing protected by seqlock is cpu_clock_offset. */
> -    seqlock_write_begin(&timers_state.vm_clock_seqlock);
> +    seqlock_write_lock(&timers_state.vm_clock_seqlock,
> +                       &timers_state.vm_clock_lock);
>      if (timers_state.cpu_ticks_enabled) {
>          timers_state.cpu_ticks_offset += cpu_get_host_ticks();
>          timers_state.cpu_clock_offset = cpu_get_clock_locked();
>          timers_state.cpu_ticks_enabled = 0;
>      }
> -    seqlock_write_end(&timers_state.vm_clock_seqlock);
> +    seqlock_write_unlock(&timers_state.vm_clock_seqlock,
> +                         &timers_state.vm_clock_lock);
>  }
> 
>  /* Correlation between real and virtual time is always going to be
> @@ -415,7 +427,8 @@ static void icount_adjust(void)
>          return;
>      }
> 
> -    seqlock_write_begin(&timers_state.vm_clock_seqlock);
> +    seqlock_write_lock(&timers_state.vm_clock_seqlock,
> +                       &timers_state.vm_clock_lock);
>      cur_time = cpu_get_clock_locked();
>      cur_icount = cpu_get_icount_locked();
> 
> @@ -439,7 +452,8 @@ static void icount_adjust(void)
>      atomic_set__nocheck(&timers_state.qemu_icount_bias,
>                          cur_icount - (timers_state.qemu_icount
>                                        << timers_state.icount_time_shift));
> -    seqlock_write_end(&timers_state.vm_clock_seqlock);
> +    seqlock_write_unlock(&timers_state.vm_clock_seqlock,
> +                         &timers_state.vm_clock_lock);
>  }
> 
>  static void icount_adjust_rt(void *opaque)
> @@ -480,7 +494,8 @@ static void icount_warp_rt(void)
>          return;
>      }
> 
> -    seqlock_write_begin(&timers_state.vm_clock_seqlock);
> +    seqlock_write_lock(&timers_state.vm_clock_seqlock,
> +                       &timers_state.vm_clock_lock);

After locking here,

>      if (runstate_is_running()) {
>          int64_t clock = REPLAY_CLOCK(REPLAY_CLOCK_VIRTUAL_RT,
>                                       cpu_get_clock_locked());

REPLAY_CLOCK can't request icount with cpu_get_icount_raw, because
it loops infinitely here:

    do {
        start = seqlock_read_begin(&timers_state.vm_clock_seqlock);
        icount = cpu_get_icount_raw_locked();
    } while (seqlock_read_retry(&timers_state.vm_clock_seqlock, start));


> @@ -500,7 +515,8 @@ static void icount_warp_rt(void)
>                              timers_state.qemu_icount_bias + warp_delta);
>      }
>      timers_state.vm_clock_warp_start = -1;
> -    seqlock_write_end(&timers_state.vm_clock_seqlock);
> +    seqlock_write_unlock(&timers_state.vm_clock_seqlock,
> +                       &timers_state.vm_clock_lock);
> 
>      if (qemu_clock_expired(QEMU_CLOCK_VIRTUAL)) {
>          qemu_clock_notify(QEMU_CLOCK_VIRTUAL);
> @@ -525,10 +541,12 @@ void qtest_clock_warp(int64_t dest)
>          int64_t deadline = qemu_clock_deadline_ns_all(QEMU_CLOCK_VIRTUAL);
>          int64_t warp = qemu_soonest_timeout(dest - clock, deadline);
> 
> -        seqlock_write_begin(&timers_state.vm_clock_seqlock);
> +        seqlock_write_lock(&timers_state.vm_clock_seqlock,
> +                           &timers_state.vm_clock_lock);
>          atomic_set__nocheck(&timers_state.qemu_icount_bias,
>                              timers_state.qemu_icount_bias + warp);
> -        seqlock_write_end(&timers_state.vm_clock_seqlock);
> +        seqlock_write_unlock(&timers_state.vm_clock_seqlock,
> +                             &timers_state.vm_clock_lock);
> 
>          qemu_clock_run_timers(QEMU_CLOCK_VIRTUAL);
>          timerlist_run_timers(aio_context->tlg.tl[QEMU_CLOCK_VIRTUAL]);
> @@ -595,10 +613,12 @@ void qemu_start_warp_timer(void)
>               * It is useful when we want a deterministic execution time,
>               * isolated from host latencies.
>               */
> -            seqlock_write_begin(&timers_state.vm_clock_seqlock);
> +            seqlock_write_lock(&timers_state.vm_clock_seqlock,
> +                               &timers_state.vm_clock_lock);
>              atomic_set__nocheck(&timers_state.qemu_icount_bias,
>                                  timers_state.qemu_icount_bias + deadline);
> -            seqlock_write_end(&timers_state.vm_clock_seqlock);
> +            seqlock_write_unlock(&timers_state.vm_clock_seqlock,
> +                                 &timers_state.vm_clock_lock);
>              qemu_clock_notify(QEMU_CLOCK_VIRTUAL);
>          } else {
>              /*
> @@ -609,12 +629,14 @@ void qemu_start_warp_timer(void)
>               * you will not be sending network packets continuously instead of
>               * every 100ms.
>               */
> -            seqlock_write_begin(&timers_state.vm_clock_seqlock);
> +            seqlock_write_lock(&timers_state.vm_clock_seqlock,
> +                               &timers_state.vm_clock_lock);
>              if (timers_state.vm_clock_warp_start == -1
>                  || timers_state.vm_clock_warp_start > clock) {
>                  timers_state.vm_clock_warp_start = clock;
>              }
> -            seqlock_write_end(&timers_state.vm_clock_seqlock);
> +            seqlock_write_unlock(&timers_state.vm_clock_seqlock,
> +                                 &timers_state.vm_clock_lock);
>              timer_mod_anticipate(timers_state.icount_warp_timer,
>                                   clock + deadline);
>          }

Pavel Dovgalyuk