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

[liu ping fan]

On Mon, Aug 5, 2013 at 9:29 PM, Paolo Bonzini <address@hidden> wrote:
>
>> In kvm mode, vm_clock may be read outside BQL.
>
> Not just in KVM mode (we will be able to use dataplane with TCG sooner
> or later), actually.
>
Oh. But this patch does not fix cpu_get_icount()'s thread-safe issue.
So currently, could I just change the commit log instead of fixing it?

Regards,
Pingfan

> Otherwise looks good!
>
> Paolo
>
>> This will make
>> timers_state --the foundation of vm_clock exposed to race condition.
>> Using private lock to protect it.
>>
>> Note in tcg mode, vm_clock still read inside BQL, so icount is
>> left without change. As for cpu_ticks in timers_state, it
>> is still protected by BQL.
>>
>> Lock rule: private lock innermost, ie BQL->"this lock"
>>
>> Signed-off-by: Liu Ping Fan <address@hidden>
>> ---
>>  cpus.c | 36 +++++++++++++++++++++++++++++-------
>>  1 file changed, 29 insertions(+), 7 deletions(-)
>>
>> 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
>> +     */
>> +    QemuSeqLock clock_seqlock;
>>      int64_t cpu_clock_offset;
>>      int32_t cpu_ticks_enabled;
>>      int64_t dummy;
>>  } TimersState;
>>
>> -TimersState timers_state;
>> +static TimersState timers_state;
>>
>>  /* Return the virtual CPU time, based on the instruction counter.  */
>>  int64_t cpu_get_icount(void)
>> @@ -132,6 +137,7 @@ int64_t cpu_get_icount(void)
>>  }
>>
>>  /* return the host CPU cycle counter and handle stop/restart */
>> +/* cpu_ticks is safely if holding BQL */
>>  int64_t cpu_get_ticks(void)
>>  {
>>      if (use_icount) {
>> @@ -156,33 +162,46 @@ int64_t cpu_get_ticks(void)
>>  int64_t cpu_get_clock(void)
>>  {
>>      int64_t ti;
>> -    if (!timers_state.cpu_ticks_enabled) {
>> -        return timers_state.cpu_clock_offset;
>> -    } else {
>> -        ti = get_clock();
>> -        return ti + timers_state.cpu_clock_offset;
>> -    }
>> +    unsigned start;
>> +
>> +    do {
>> +        start = seqlock_read_begin(&timers_state.clock_seqlock);
>> +        if (!timers_state.cpu_ticks_enabled) {
>> +            ti = timers_state.cpu_clock_offset;
>> +        } else {
>> +            ti = get_clock();
>> +            ti += timers_state.cpu_clock_offset;
>> +        }
>> +    } while (seqlock_read_check(&timers_state.clock_seqlock, start);
>> +
>> +    return ti;
>>  }
>>
>>  /* enable cpu_get_ticks() */
>>  void cpu_enable_ticks(void)
>>  {
>> +    /* Here, the really thing protected by seqlock is cpu_clock. */
>> +    seqlock_write_lock(&timers_state.clock_seqlock);
>>      if (!timers_state.cpu_ticks_enabled) {
>>          timers_state.cpu_ticks_offset -= cpu_get_real_ticks();
>>          timers_state.cpu_clock_offset -= get_clock();
>>          timers_state.cpu_ticks_enabled = 1;
>>      }
>> +    seqlock_write_unlock(&timers_state.clock_seqlock);
>>  }
>>
>>  /* disable cpu_get_ticks() : the clock is stopped. You must not call
>>     cpu_get_ticks() after that.  */
>>  void cpu_disable_ticks(void)
>>  {
>> +    /* Here, the really thing protected by seqlock is cpu_clock. */
>> +    seqlock_write_lock(&timers_state.clock_seqlock);
>>      if (timers_state.cpu_ticks_enabled) {
>>          timers_state.cpu_ticks_offset = cpu_get_ticks();
>>          timers_state.cpu_clock_offset = cpu_get_clock();
>>          timers_state.cpu_ticks_enabled = 0;
>>      }
>> +    seqlock_write_unlock(&timers_state.clock_seqlock);
>>  }
>>
>>  /* Correlation between real and virtual time is always going to be
>> @@ -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);
>>      vmstate_register(NULL, 0, &vmstate_timers, &timers_state);
>>      if (!option) {
>>          return;
>> --
>> 1.8.1.4
>>
>>
>