Re: [Qemu-devel] [RFC 7/8] cpu-exec-common: Introduce async_safe_run_on_cpu()

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 01/07/16 19:29, Alvise Rigo wrote:
> Hi Sergey,
>
> On Mon, Jun 20, 2016 at 12:28 AM, Sergey Fedorov
> <sergey.fedorov@linaro.org> wrote:
>> diff --git a/cpu-exec-common.c b/cpu-exec-common.c
>> index 8184e0662cbd..3056324738f8 100644
>> --- a/cpu-exec-common.c
>> +++ b/cpu-exec-common.c
>> @@ -25,6 +25,7 @@
>>
>>  bool exit_request;
>>  CPUState *tcg_current_cpu;
>> +int tcg_pending_cpus;
>>
>>  /* exit the current TB, but without causing any exception to be raised */
>>  void cpu_loop_exit_noexc(CPUState *cpu)
>> @@ -78,6 +79,15 @@ void cpu_loop_exit_restore(CPUState *cpu, uintptr_t pc)
>>      siglongjmp(cpu->jmp_env, 1);
>>  }
>>
>> +static int safe_work_pending;
>> +
>> +void wait_safe_cpu_work(void)
>> +{
>> +    while (atomic_mb_read(&safe_work_pending) > 0) {
>> +        wait_cpu_work();
>> +    }
>> +}
>> +
> Is this piece of code deadlock-safe once we are in mttcg mode?

It is supposed to be deadlock-safe.

> What happens when two threads call simultaneously async_safe_run_on_cpu?
>

In this case each thread will roughly:
 - exit its execution loop;
 - take BQL;
 - decrement 'tcg_pending_cpus', signal 'qemu_work_cond' if zero;
 - start processing its work queue;
 - encountering safe work wait on 'qemu_work_cond' for
'tcg_pending_cpus' to become zero;
 - reacquire BQL;
 - process the safe work;
 - decrement 'safe_work_pending', signal 'qemu_work_cond' if zero;
 - when finished processing work, wait on 'qemu_work_cond' for
'safe_work_pending' to become zero;
 - reacquire BQL;
 - continue execution (releasing BQL).

Hope this will help.

Kind regards,
Sergey.