Messages in this thread Patch in this message | | | Date | Mon, 15 Dec 2008 17:51:12 +0800 | From | Li Zefan <> | Subject | Re: [PATCH] sched: fix another race when reading /proc/sched_debug |
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>>> Can't we detect a dead task-group and skip those instead of adding this >>> global lock? >>> >> I tried it, but I don't think it's feasable, without lock syncronization: >> >> | print_cfs_rq() >> | check task_group is dead >> cgroup_diput() | >> .. | >> mark task_group as dead | >> .. | >> kfree(cgrp) | >> | call cgroup_path() > > rcu free cgrp >
I got your point, thanks.
Another way is use css_tryget(), and thus can avoid touching cgroup.c and adding synchronize_rcu(). css_tryget() is proposed by Kamezawa but I think won't be available until 2.6.29.
Anyway, here is the fix. I'll post a complete version with changelog when we agree on how to fix it.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com> --- kernel/cgroup.c | 6 ++++++ kernel/sched_debug.c | 17 +++++++++++++++-- 2 files changed, 21 insertions(+), 2 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c index fe00b3b..3c54d1b 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -624,6 +624,12 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode) * created the cgroup */ deactivate_super(cgrp->root->sb); + /* + * Some subsystems (cpu cgroup) might still be able to + * accessing the cgroup in rcu section. + */ + synchronize_rcu(); + kfree(cgrp); } iput(inode); diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c index 26ed8e3..174c072 100644 --- a/kernel/sched_debug.c +++ b/kernel/sched_debug.c @@ -127,8 +127,14 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq) if (tg) cgroup = tg->css.cgroup; - if (cgroup) + if (cgroup) { + /* + * This task_group is dead or we race with cgroup creating. + */ + if (cgroup_is_removed(cgroup) || !cgroup->dentry) + return; cgroup_path(cgroup, path, sizeof(path)); + } SEQ_printf(m, "\ncfs_rq[%d]:%s\n", cpu, path); #else @@ -181,8 +187,15 @@ void print_rt_rq(struct seq_file *m, int cpu, struct rt_rq *rt_rq) if (tg) cgroup = tg->css.cgroup; - if (cgroup) + if (cgroup) { + /* + * This task_group is dead or we race with cgroup creating. + */ + if (cgroup_is_removed(cgroup) || !cgroup->dentry) + return; + cgroup_path(cgroup, path, sizeof(path)); + } SEQ_printf(m, "\nrt_rq[%d]:%s\n", cpu, path); #else -- 1.5.4.rc3
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