[lttng-dev] Deaklock in liblttng-ust

[changz]

On 9/17/2012 21:33 PM, Mathieu Desnoyers wrote:
> * changz (zheng.chang at emc.com) wrote:
>> ......
>>
>> The child process calls _fini when it calls API exit. It gets hung and
>> meanwhile the parent is waiting for its termination.
>> I think the whole life-cycle of the process should be considered. The
>> parent's waiting in critical region is dangerous.
>> Is it possible to refine the critical region with smaller fineness?
>>
>> What do you think?
> Hrm, yes you're right. I'm looking into it.
>
> The main issue is that get_wait_shm() bypass the fork() wrapper (with
> lttng_ust_nest_count), which is responsible for holding the UST mutex
> across fork(). Therefore, when exiting the context of the child process,
> we execute the destructor, which try to grab the UST mutex, which might
> be in pretty much any state.
>
> Given that we don't want this process to try to register to
> lttng-sessiond (because this is internal to lttng-ust), we might want to
> let it skip the destructor execution. This would actually be the easiest
> way out.
>
> Does the follow patch fix the issue for you ?
>
> diff --git a/liblttng-ust/lttng-ust-comm.c b/liblttng-ust/lttng-ust-comm.c
> index be64acd..596fd7d 100644
> --- a/liblttng-ust/lttng-ust-comm.c
> +++ b/liblttng-ust/lttng-ust-comm.c
> @@ -616,9 +616,9 @@ int get_wait_shm(struct sock_info *sock_info, size_t mmap_size)
>   			ret = ftruncate(wait_shm_fd, mmap_size);
>   			if (ret) {
>   				PERROR("ftruncate");
> -				exit(EXIT_FAILURE);
> +				_exit(EXIT_FAILURE);
>   			}
> -			exit(EXIT_SUCCESS);
> +			_exit(EXIT_SUCCESS);
>   		}
>   		/*
>   		 * For local shm, we need to have rw access to accept
Yes, it works.
Just a reminder, here arefour callings of exit in child's path in my git 
repository.

Thanks & BR
-Zheng



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