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Re: [PATCH v3 2/2] posix: Use posix_spawn on system
* Adhemerval Zanella:
> On 30/11/2018 13:21, Florian Weimer wrote:
>> * Adhemerval Zanella:
>>> On 29/11/2018 15:37, Florian Weimer wrote:
>>>> * Adhemerval Zanella:
>>>>> +/* We have to and actually can handle cancelable system(). The big
>>>>> + problem: we have to kill the child process if necessary. To do
>>>>> + this a cleanup handler has to be registered and it has to be able
>>>>> + to find the PID of the child. The main problem is to reliable have
>>>>> + the PID when needed. It is not necessary for the parent thread to
>>>>> + return. It might still be in the kernel when the cancellation
>>>>> + request comes. Therefore we have to use the clone() calls ability
>>>>> + to have the kernel write the PID into the user-level variable. */
>>>> This comment does not look relevant to me anymore.
>>> I think it still worth to mention glibc system aims to be thread-safe,
>>> which requires restore the signal dispositions for SIGINT and SIGQUIT
>>> correctly and to deal with cancellation by terminating the child process.
>>> +/* This system implementation aims to be thread-safe, which requires restore
>>> + the signal dispositions for SIGINT and SIGQUIT correctly and to deal with
>>> + cancellation by terminating the child process. */
>> I don't think you restore SIGINT and SIGQUIT correctly for concurrent
>> system calls. This is what the ADD_REF code in the old version
>> attempted to do.
> It is not strictly incorrect, although Linux sigaction is not really
> thread-safe (due the copy in/out kernel sigaction structure). And it
> is indeed not optional, and I agree that relying on this benign data race
> behaviour is not correct. Below it is an updated patch with ref counter
It's not about the data race, it is about the higher-level race
condition. The problem is that the first thread to enter system and
capture the original signal state may not be the last to leave system
and restore things.
> +static void
> +cancel_handler (void *arg)
> + struct cancel_handler_args *args = (struct cancel_handler_args *) (arg);
> + __kill_noerrno (args->pid, SIGKILL);
> + TEMP_FAILURE_RETRY (__waitpid (args->pid, NULL, 0));
One last question (I promise): Should this be the nocancel variant?