linux-kernel - [PATCH] cputime: avoid do_sys_times() races with __exit

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Message-ID: <20091123101612.GC25978@dhcp-lab-161.englab.brq.redhat.com>
Date:	Mon, 23 Nov 2009 11:16:12 +0100
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Spencer Candland <spencer@...ehost.com>,
	Américo Wang <xiyou.wangcong@...il.com>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Balbir Singh <balbir@...ibm.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Subject: [PATCH] cputime: avoid do_sys_times() races with __exit_signal()

When process exit in the middle of thread_group_cputime() loop, {u,s}time
values will be accounted twice. One time - in all threads loop, second - in
__exit_signal(). This make sys_times() return values bigger then they
are in real. Next consecutive call to sys_times() can return correct values,
so we have {u,s}time decrease.

To fix use sighand->siglock in do_sys_times().

This is partial fix for problem of utime/stime values decreasing described
in this thread:

http://lkml.org/lkml/2009/11/3/522

To fully fix the problem, we need second fix for mishmash between
task_{u,s}time() and tsk->{u,s}time. It's not clear now how this mishmash
should be fixed.

Signed-off-by: Stanislaw Gruszka <sgruszka@...hat.com>
---
 kernel/sys.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index ce17760..8be5b75 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -914,8 +914,8 @@ void do_sys_times(struct tms *tms)
 	struct task_cputime cputime;
 	cputime_t cutime, cstime;
 
-	thread_group_cputime(current, &cputime);
 	spin_lock_irq(&current->sighand->siglock);
+	thread_group_cputime(current, &cputime);
 	cutime = current->signal->cutime;
 	cstime = current->signal->cstime;
 	spin_unlock_irq(&current->sighand->siglock);
-- 
1.6.2.5

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