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Date:	Mon, 28 Jul 2014 20:38:29 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org,
	linux-arch@...r.kernel.org, linux-security-module@...r.kernel.org,
	Alexei Starovoitov <ast@...mgrid.com>, hpa@...or.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ

The RCU context tracking code requires that arch code call
user_exit() on any entry into kernel code if TIF_NOHZ is set.  This
patch adds a check for TIF_NOHZ and a comment to the syscall entry
tracing code.

The main purpose of this patch is to make the code easier to follow:
one can read the body of user_exit and of every function it calls
without finding any explanation of why it's called for traced
syscalls but not for untraced syscalls.  This makes it clear when
user_exit() is necessary.

Cc: Frederic Weisbecker <fweisbec@...il.com>
Signed-off-by: Andy Lutomirski <luto@...capital.net>
---
 arch/x86/kernel/ptrace.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 39296d2..bbf338a 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -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();
 
 	/*
 	 * If we stepped into a sysenter/syscall insn, it trapped in
-- 
1.9.3

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