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x86, entry: Only call user_exit if TIF_NOHZ

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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@gmail.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/0b13e0e24ec0307d67ab7a23b58764f6b1270116.1409954077.git.luto@amacapital.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
This commit is contained in:
Andy Lutomirski 2014-09-05 15:13:53 -07:00 committed by H. Peter Anvin
parent 81f49a8fd7
commit fd143b210e
1 changed files with 6 additions and 1 deletions
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7
arch/x86/kernel/ptrace.c
View file

@ -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