Re: [dpdk-dev] [PATCH v2] eal: fix race in ctrl thread creation

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> 
> As reported by Luc, there is a race where the barrier is destroyed by one
> thread, while the other thread did not yet leave pthread_barrier_wait.
Please correct me if I am wrong. We are using the pthread_barrier to

1) know when to free 'params'.
2) set the thread affinity before the thread starts running its function

I think this patch is fine. But, it can be simplified.

> 
> This patch fixes the race condition by adding an atomic counter to ensure
> that the barrier is destroyed only it is not used by any thread.
> 
> Fixes: 3a0d465d4c53 ("eal: fix use-after-free on control thread creation")
> Cc: jianfeng.tan@intel.com
> Cc: stable@dpdk.org
> 
> Reported-by: Luc Pelletier <lucp.at.work@gmail.com>
> Signed-off-by: David Marchand <david.marchand@redhat.com>
> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> ---
> 
> Hi Luc,
> 
> Thank you for reporting this problem and submitting the patch.
> I think the issue can be fixed without any loop, like in this patch. What do
> you think?
> 
> Regards,
> Olivier
> 
> 
>  lib/librte_eal/common/eal_common_thread.c | 38 +++++++++++++----------
>  1 file changed, 21 insertions(+), 17 deletions(-)
> 
> diff --git a/lib/librte_eal/common/eal_common_thread.c
> b/lib/librte_eal/common/eal_common_thread.c
> index 73a055902a..891f825e87 100644
> --- a/lib/librte_eal/common/eal_common_thread.c
> +++ b/lib/librte_eal/common/eal_common_thread.c
> @@ -170,11 +170,11 @@ struct rte_thread_ctrl_params {
>  	void *(*start_routine)(void *);
>  	void *arg;
>  	pthread_barrier_t configured;
We can get rid of the barrier variable. The 'barrier_refcnt' is enough to know when to free the memory.
Setting the thread affinity can be moved to 'ctrl_thread_init' function.

> +	unsigned int barrier_refcnt;
>  };
> 
>  static void *ctrl_thread_init(void *arg)  {
> -	int ret;
>  	struct internal_config *internal_conf =
>  		eal_get_internal_configuration();
>  	rte_cpuset_t *cpuset = &internal_conf->ctrl_cpuset; @@ -184,8
> +184,9 @@ static void *ctrl_thread_init(void *arg)
> 
>  	__rte_thread_init(rte_lcore_id(), cpuset);
> 
> -	ret = pthread_barrier_wait(&params->configured);
> -	if (ret == PTHREAD_BARRIER_SERIAL_THREAD) {
> +	pthread_barrier_wait(&params->configured);
> +	if (__atomic_sub_fetch(&params->barrier_refcnt, 1,
> +				__ATOMIC_ACQ_REL) == 0) {
>  		pthread_barrier_destroy(&params->configured);
>  		free(params);
>  	}
> @@ -210,15 +211,17 @@ rte_ctrl_thread_create(pthread_t *thread, const
> char *name,
> 
>  	params->start_routine = start_routine;
>  	params->arg = arg;
> -
> -	pthread_barrier_init(&params->configured, NULL, 2);
> -
> -	ret = pthread_create(thread, attr, ctrl_thread_init, (void *)params);
> +	__atomic_store_n(&params->barrier_refcnt, 2,
> __ATOMIC_RELEASE);
We can use __ATOMIC_RELAXED or just a simple assignment as pthread_create ensures that all the earlier memory writes are visible to the 'ctrl_thread_init'.

> +	ret = pthread_barrier_init(&params->configured, NULL, 2);
>  	if (ret != 0) {
>  		free(params);
>  		return -ret;
>  	}
> 
> +	ret = pthread_create(thread, attr, ctrl_thread_init, (void *)params);
> +	if (ret != 0)
> +		goto fail;
> +
>  	if (name != NULL) {
>  		ret = rte_thread_setname(*thread, name);
>  		if (ret < 0)
> @@ -227,25 +230,26 @@ rte_ctrl_thread_create(pthread_t *thread, const
> char *name,
>  	}
> 
>  	ret = pthread_setaffinity_np(*thread, sizeof(*cpuset), cpuset);
I think this can be moved to 'ctrl_thread_init'.

> -	if (ret)
> -		goto fail;
> +	if (ret != 0)
> +		goto fail_cancel;
> 
> -	ret = pthread_barrier_wait(&params->configured);
> -	if (ret == PTHREAD_BARRIER_SERIAL_THREAD) {
> +	pthread_barrier_wait(&params->configured);
> +	if (__atomic_sub_fetch(&params->barrier_refcnt, 1,
> +				__ATOMIC_ACQ_REL) == 0) {
>  		pthread_barrier_destroy(&params->configured);
>  		free(params);
>  	}
> 
>  	return 0;
> 
> -fail:
> -	if (PTHREAD_BARRIER_SERIAL_THREAD ==
> -	    pthread_barrier_wait(&params->configured)) {
> -		pthread_barrier_destroy(&params->configured);
> -		free(params);
> -	}
> +fail_cancel:
>  	pthread_cancel(*thread);
>  	pthread_join(*thread, NULL);
> +
> +fail:
> +	pthread_barrier_destroy(&params->configured);
> +	free(params);
> +
>  	return -ret;
>  }
> 
> --
> 2.29.2