Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ
From: Oleg Nesterov
Date: Tue Jul 29 2014 - 15:34:51 EST
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.
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