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

From: Andy Lutomirski
Date: Mon Jul 28 2014 - 23:40:00 EST

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@xxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
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

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