RE: [PATCH v2] eal: fix thread race in control thread creation

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

> -----Original Message-----
> From: Tyler Retzlaff <roretzla@linux.microsoft.com>
> Sent: Wednesday, March 1, 2023 3:10 PM
> To: dev@dpdk.org; david.marchand@redhat.com
> Cc: thomas@monjalon.net; Tyler Retzlaff <roretzla@linux.microsoft.com>
> Subject: [PATCH v2] eal: fix thread race in control thread creation
> 
> When ctrl_thread_init transitions params->ctrl_thread_status from
> CTRL_THREAD_LAUNCHING the creating thread and new thread may run
> concurrently leading to unsynchronized access to params.
IMO, the code will be simpler if we did not free 'params' in 'rte_thread_create_control'/'rte_ctrl_thread_create'. We could avoid creating the local copies of start_routine and the arg.
See more comments below.

> 
> This permits races for both the failure and success paths after ctrl_thread_status
> is stored.
>   * params->ret may be loaded in ctrl_thread_init failure path
>   * params->arg may be loaded in ctrl_thread_start or
>     control_thread_start when calling start_routine.
> 
> For ctrl_thread_init remove the params->ret load and just return 1 since it is
> only interpreted as a indicator of success / failure of ctrl_thread_init.
> 
> For {ctrl,control}_thread_start store param->arg in stack allocated storage prior
> to calling ctrl_thread_init and use the copy when calling start_routine.
> 
> For control_thread_start if ctrl_thread_init fails just return 0 instead of loading
> params->ret, since the value returned is unused when ctrl_thread_status is set
> to CTRL_THREAD_ERROR when ctrl_thread_init fails.
> 
> Fixes: 878b7468eacb ("eal: add platform agnostic control thread API")
> 
> Signed-off-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
> Reviewed-by: David Marchand <david.marchand@redhat.com>
> ---
>  lib/eal/common/eal_common_thread.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/eal/common/eal_common_thread.c
> b/lib/eal/common/eal_common_thread.c
> index edb9d4e..079a385 100644
> --- a/lib/eal/common/eal_common_thread.c
> +++ b/lib/eal/common/eal_common_thread.c
> @@ -256,7 +256,7 @@ static int ctrl_thread_init(void *arg)
>  	if (params->ret != 0) {
>  		__atomic_store_n(&params->ctrl_thread_status,
>  			CTRL_THREAD_ERROR, __ATOMIC_RELEASE);
> -		return params->ret;
> +		return 1;
>  	}
> 
>  	__atomic_store_n(&params->ctrl_thread_status,
> @@ -268,23 +268,25 @@ static int ctrl_thread_init(void *arg)  static void
> *ctrl_thread_start(void *arg)  {
>  	struct rte_thread_ctrl_params *params = arg;
> +	void *start_arg = params->arg;
>  	void *(*start_routine)(void *) = params->u.ctrl_start_routine;
These copies can be avoided, code will be much simpler

> 
>  	if (ctrl_thread_init(arg) != 0)
>  		return NULL;
> 
> -	return start_routine(params->arg);
> +	return start_routine(start_arg);
We can free 'params' here after 'start_routine' returns.

>  }
> 
>  static uint32_t control_thread_start(void *arg)  {
>  	struct rte_thread_ctrl_params *params = arg;
> +	void *start_arg = params->arg;
>  	rte_thread_func start_routine = params->u.control_start_routine;
> 
>  	if (ctrl_thread_init(arg) != 0)
> -		return params->ret;
> +		return 0;
> 
> -	return start_routine(params->arg);
> +	return start_routine(start_arg);
>  }
> 
>  int
> --
> 1.8.3.1