>+static const char *freezer_state_strs[] = {
>+ "RUNNING\n",
>+ "FREEZING\n" ,
>+ "FROZEN\n"
>+};
I think it might be cleaner to not include the \n characters in this array.
>+static inline int cgroup_frozen(struct task_struct *task)
>+{
>+ struct cgroup *cgroup = task_cgroup(task, freezer_subsys_id);
>+ struct freezer *freezer = cgroup_freezer(cgroup);
>+ enum freezer_state state;
>+
>+ spin_lock(&freezer->lock);
>+ state = freezer->state;
>+ spin_unlock(&freezer->lock);
>+
>+ return (state == STATE_FROZEN);
>+}
You need to be in an RCU critical section or else hold task_lock() in
order to dereference the cgroup returned from task_cgroup()
I'm not sure that you need to take freezer->lock here - you're just
reading a single word.
>+
>+ if (!capable(CAP_SYS_ADMIN))
>+ return ERR_PTR(-EPERM);
>+
Why does everyone keep throwing calls to check CAP_SYS_ADMIN into
their cgroup create callbacks? You have to be root in order to mount a
cgroups hierarchy in the first place, and filesystem permissions will
control who can create new cgroups.
>+static int freezer_can_attach(struct cgroup_subsys *ss,
>+ struct cgroup *new_cgroup,
>+ struct task_struct *task)
>+{
>+ struct freezer *freezer = cgroup_freezer(new_cgroup);
>+ int retval = 0;
>+
>+ if (freezer->state == STATE_FROZEN)
>+ retval = -EBUSY;
>+
>+ return retval;
>+}
You should comment here that 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 ssize_t freezer_write(struct cgroup *cgroup,
>+ struct cftype *cft,
>+ struct file *file,
>+ const char __user *userbuf,
>+ size_t nbytes, loff_t *unused_ppos)
>+{
>+ char *buffer;
>+ int retval = 0;
>+ enum freezer_state goal_state;
>+
>+ if (nbytes >= PATH_MAX)
>+ return -E2BIG;
>+
>+ /* +1 for nul-terminator */
>+ buffer = kmalloc(nbytes + 1, GFP_KERNEL);
>+ if (buffer == NULL)
>+ return -ENOMEM;
Given that you're copying a string whose maximum valid length is
"FREEZING" you don't really need to use a dynamically-allocated
buffer.
But I really ought to provide a write_string() method that handles
this kind of copying on behalf of cgroup subsystems, the way it
already does for 64-bit ints.
>+ if (strcmp(buffer, "RUNNING") == 0)
>+ goal_state = STATE_RUNNING;
>+ else if (strcmp(buffer, "FROZEN") == 0)
>+ goal_state = STATE_FROZEN;
Would it make sense to compare against the strings you already have in
the array earlier in the file?
Paul
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