linux-kernel - Re: [PATCH] cpuset: make ntasks to be a monotonic increasing value

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Message-Id: <20080731072355.b582b2d6.pj@sgi.com>
Date:	Thu, 31 Jul 2008 07:23:55 -0500
From:	Paul Jackson <pj@....com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	akpm@...ux-foundation.org, menage@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpuset: make ntasks to be a monotonic increasing value

I agree that in theory, this kernel/cpuset.c update_tasks_nodemask()
loop could loop forever, and that by forcing ntasks to keep increasing
monotonically, this guarantees that it cannot loop forever.

I also agree that no known exploit of this exists, and doubt that
any could be created.

I did find the added code logic to be a tad more difficult to read
than I'd like.  How about the following patch, instead:

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

--- 2.6.25-mm1.orig/kernel/cpuset.c	2008-07-31 07:05:23.000000000 -0500
+++ 2.6.25-mm1/kernel/cpuset.c	2008-07-31 07:13:48.000000000 -0500
@@ -880,6 +880,7 @@ static int update_nodemask(struct cpuset
 	struct task_struct *p;
 	struct mm_struct **mmarray;
 	int i, n, ntasks;
+	int prev_ntasks = 0;
 	int migrate;
 	int fudge;
 	int retval;
@@ -939,7 +940,9 @@ static int update_nodemask(struct cpuset
 	 */
 	while (1) {
 		ntasks = cgroup_task_count(cs->css.cgroup);  /* guess */
+		ntasks = max(ntasks, prev_ntasks);	/* keep increasing */
 		ntasks += fudge;
+		prev_ntasks = ntasks;
 		mmarray = kmalloc(ntasks * sizeof(*mmarray), GFP_KERNEL);
 		if (!mmarray)
 			goto done;


-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@....com> 1.940.382.4214
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