Messages in this thread | | | Date | Tue, 21 Oct 2008 17:29:23 +0800 | From | Li Zefan <> | Subject | Re: [PATCH 2/4] freezer_cg: remove redundant check in freezer_can_attach() |
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Cedric Le Goater wrote: > Li Zefan wrote: >> It is sufficient to check if @task is frozen, and no need to check if >> the original freezer is frozen. > > hmm, a task being frozen does not mean that its freezer cgroup is > frozen.
If a task has being frozen, then can_attach() returns -EBUSY at once. If a task isn't frozen, then we have orig_freezer->state == THAWED.
So we need to check the state of the task only.
> So the extra check seems valid but looking at the comment : > > /* > * The call to cgroup_lock() in the freezer.state write method prevents > * a write to that file racing against an attach, and hence the > * can_attach() result will remain valid until the attach completes. > */ > static int freezer_can_attach(struct cgroup_subsys *ss, > > how do we know that the task_freezer(task), which is not protected by > the cgroup_lock(), is not going to change its state to CGROUP_FROZEN > it looks racy. >
Since freezer_can_attach() is called by cgroup core with cgroup_lock held, there is no race with task attaching and state changing.
> C. > >> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com> >> --- >> kernel/cgroup_freezer.c | 16 +++++++--------- >> 1 files changed, 7 insertions(+), 9 deletions(-) >> >> diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c >> index 7f54d1c..6fadafe 100644 >> --- a/kernel/cgroup_freezer.c >> +++ b/kernel/cgroup_freezer.c >> @@ -162,9 +162,13 @@ static int freezer_can_attach(struct cgroup_subsys *ss, >> struct task_struct *task) >> { >> struct freezer *freezer; >> - int retval; >> >> - /* Anything frozen can't move or be moved to/from */ >> + /* >> + * Anything frozen can't move or be moved to/from. >> + * >> + * Since orig_freezer->state == FROZEN means that @task has been >> + * frozen, so it's sufficient to check the latter condition. >> + */ >> >> if (is_task_frozen_enough(task)) >> return -EBUSY; >> @@ -173,13 +177,7 @@ static int freezer_can_attach(struct cgroup_subsys *ss, >> if (freezer->state == CGROUP_FROZEN) >> return -EBUSY; >> >> - retval = 0; >> - task_lock(task); >> - freezer = task_freezer(task); >> - if (freezer->state == CGROUP_FROZEN) >> - retval = -EBUSY; >> - task_unlock(task); >> - return retval; >> + return 0; >> } >> >> static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task) > >
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