Subject: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ

From: [email-masked] (Oleg Nesterov)
Date: Tue, 29 Jul 2014 21:32:32 +0200

On 07/28, Andy Lutomirski wrote:
>
> @@ -1449,7 +1449,12 @@ long syscall_trace_enter(struct pt_regs *regs)
>  {
>  	long ret = 0;
>  
> -	user_exit();
> +	/*
> +	 * If TIF_NOHZ is set, we are required to call user_exit() before
> +	 * doing anything that could touch RCU.
> +	 */
> +	if (test_thread_flag(TIF_NOHZ))
> +		user_exit();

Personally I still think this change just adds more confusion, but I leave
this to you and Frederic.

It is not that "If TIF_NOHZ is set, we are required to call user_exit()", we
need to call user_exit() just because we enter the kernel. TIF_NOHZ is just
the implementation detail which triggers this slow path.

At least it should be correct, unless I am confused even more than I think.

Oleg.