linux-kernel - [PATCH -mm 3/4] cputimers: thread_group

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Message-ID: <20100329181359.GD16356@redhat.com>
Date:	Mon, 29 Mar 2010 20:13:59 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Americo Wang <xiyou.wangcong@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Roland McGrath <roland@...hat.com>,
	Spencer Candland <spencer@...ehost.com>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH -mm 3/4] cputimers: thread_group_times: make it rcu-safe

thread_group_times() relies on ->siglock hold by the caller when it
updates sig->prev_Xtime members. Change the code to take cputimer.lock
around the assignment.

This makes it rcu-safe and fixes the theoretical race. wait_task_zombie()
calls it lockless and thus can race with do_task_stat().

Note: we are taking cputimer.lock under ->siglock but this dependency
is not new, thread_group_cputimer() already does this.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---

 kernel/sched.c |    3 +++
 1 file changed, 3 insertions(+)

--- 34-rc1/kernel/sched.c~cpuacct_3_thread_group_times_dont_rely_on_siglock	2010-03-29 19:44:25.000000000 +0200
+++ 34-rc1/kernel/sched.c	2010-03-29 19:46:21.000000000 +0200
@@ -3449,6 +3449,7 @@ void thread_group_times(struct task_stru
 	struct signal_struct *sig = p->signal;
 	struct task_cputime cputime;
 	cputime_t rtime, utime, total;
+	unsigned long flags;
 
 	thread_group_cputime(p, &cputime);
 
@@ -3464,9 +3465,11 @@ void thread_group_times(struct task_stru
 	} else
 		utime = rtime;
 
+	spin_lock_irqsave(&sig->cputimer.lock, flags);
 	sig->prev_utime = max(sig->prev_utime, utime);
 	sig->prev_stime = max(sig->prev_stime,
 			      cputime_sub(rtime, sig->prev_utime));
+	spin_unlock_irqrestore(&sig->cputimer.lock, flags);
 
 	*ut = sig->prev_utime;
 	*st = sig->prev_stime;

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