On 10/16, Tejun Heo wrote:
>
> +/**
> + * freezer_count - tell freezer to stop ignoring %current
> + *
> + * Undo freezer_do_not_count(). It tells freezers that %current should be
> + * considered again and tries to freeze if freezing condition is already in
> + * effect.
> */
> static inline void freezer_count(void)
> {
> current->flags &= ~PF_FREEZER_SKIP;
> + /*
> + * If freezing is in progress, the following paired with smp_mb()
> + * in freezer_should_skip() ensures that either we see %true
> + * freezing() or freezer_should_skip() sees !PF_FREEZER_SKIP.
> + */
> + smp_mb();
> try_to_freeze();
I agree, this looks like a bug fix.
> -static inline int freezer_should_skip(struct task_struct *p)
> +static inline bool freezer_should_skip(struct task_struct *p)
> {
> - return !!(p->flags & PF_FREEZER_SKIP);
> + /*
> + * The following smp_mb() paired with the one in freezer_count()
> + * ensures that either freezer_count() sees %true freezing() or we
> + * see cleared %PF_FREEZER_SKIP and return %false. This makes it
> + * impossible for a task to slip frozen state testing after
> + * clearing %PF_FREEZER_SKIP.
> + */
> + smp_mb();
> + return p->flags & PF_FREEZER_SKIP;
> }
I am not sure we really need smp_mb() here. Speaking of cgroup_freezer,
it seems that a single mb() after "->state = CGROUP_FREEZING" should be
enough.
But even if I am right, I agree that it looks better in freezer_should_skip()
and this is more robust.
So I think the patch is fine and fixes the bug.
We probably have another similar race. If ptrace_stop()->may_ptrace_stop()
returns false, the task does
__set_current_state(TASK_RUNNING);
// no mb in between
try_to_freeze();
And this can race with task_is_stopped_or_traced() check in the same way.
(of course this is only theoretical).
do_signal_stop() is probably fine, we can rely on ->siglock.
Oleg.
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