On 01/07/16 19:29, Alvise Rigo wrote:
> Hi Sergey,
>
> On Mon, Jun 20, 2016 at 12:28 AM, Sergey Fedorov
> <sergey.fedo...@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.
