On 03/21, Mandeep Singh Baines wrote:
>
> Oleg Nesterov (oleg@xxxxxxxxxx) wrote:
> > +/*
> > + * rcu-safe, but should start at ->group_leader.
> > + * thread_group_leader(g) protects against the race with exec which
> > + * removes the leader from list.
> > + * smp_rmb() pairs with implicit mb() implied by unlock + lock in
> > + * de_thread()->release_task() path.
> > + */
> > +#define while_each_thread_rcu(g, t) \
> > + while ((t = next_thread(t)) != g && \
> > + ({ smp_rmb(); thread_group_leader(g); }))
> > +
>
> Couldn't you miss the exec_thread if:
>
> t = exec_thread && !thread_group_leader(g)
Yes, we already discussed this, iirc.
I was going to write that this is fine, but then I changed my mind.
Indeed, it is not good while_each_thread_rcu() can miss the new leader.
> Could we change do_prlimit()? Especially since its slow path.
But do_prlimit() is correct. It sees the unhashed task under tasklist,
task->group_leader should be correct.
> I really like you're earlier solution (ignoring barrier):
>
> #define while_each_thread(g, t) \
> while (t->group_leader == g->group_leader && (t = next_thread(t)) != g)
The problem is "ignoring barrier and races".
OK, I'll try to think again.
Oleg.
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