Re: psi_trigger_poll() is completely broken

[Linus Torvalds]

On Wed, Jan 5, 2022 at 11:07 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> Whoever came up with that stupid "replace existing trigger with a
> write()" model should feel bad. It's garbage, and it's actively buggy
> in multiple ways.

What are the users? Can we make the rule for -EBUSY simply be that you
can _install_ a trigger, but you can't replace an existing one (except
with NULL, when you close it).

That would fix the poll() lifetime issue, and would make the
psi_trigger_replace() races fairly easy to fix - just use

        if (cmpxchg(trigger_ptr, NULL, new) != NULL) {
                ... free 'new', return -EBUSY ..

to install the new one, instead of

        rcu_assign_pointer(*trigger_ptr, new);

or something like that. No locking necessary.

But I assume people actually end up re-writing triggers, because
people are perverse and have taken advantage of this completely broken
API.

               Linus