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Date:   Mon, 28 Jul 2014 20:58:03 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Andy Lutomirski <luto@amacapital.net>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc:     linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        Will Drewry <wad@chromium.org>, x86@kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-mips@linux-mips.org,
        linux-arch@vger.kernel.org, linux-security-module@vger.kernel.org,
        Alexei Starovoitov <ast@plumgrid.com>, hpa@zytor.com
Subject: TIF_NOHZ can escape nonhz mask? (Was: [PATCH v3 6/8] x86: Split
        syscall_trace_enter into two phases)
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References: <cover.1405992946.git.luto@amacapital.net> <3f649f5658a163645e3ce15156176c325283762e.1405992946.git.luto@amacapital.net> <20140728173723.GA20993@redhat.com>
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Off-topic, but...

On 07/28, Oleg Nesterov wrote:
>
> But we should always call user_exit() unconditionally?

Frederic, don't we need the patch below? In fact clear_() can be moved
under "if ()" too. and probably copy_process() should clear this flag...

Or. __context_tracking_task_switch() can simply do

	 if (context_tracking_cpu_is_enabled())
	 	set_tsk_thread_flag(next, TIF_NOHZ);
	 else
	 	clear_tsk_thread_flag(next, TIF_NOHZ);

and then we can forget about copy_process(). Or I am totally confused?


I am also wondering if we can extend user_return_notifier to handle
enter/exit and kill TIF_NOHZ.

Oleg.

--- x/kernel/context_tracking.c
+++ x/kernel/context_tracking.c
@@ -202,7 +202,8 @@ void __context_tracking_task_switch(stru
 				    struct task_struct *next)
 {
 	clear_tsk_thread_flag(prev, TIF_NOHZ);
-	set_tsk_thread_flag(next, TIF_NOHZ);
+	if (context_tracking_cpu_is_enabled())
+		set_tsk_thread_flag(next, TIF_NOHZ);
 }
 
 #ifdef CONFIG_CONTEXT_TRACKING_FORCE