Re: [PATCH] tracing/user_events: Run BPF program if attached

[Linus Torvalds]

On Tue, May 16, 2023 at 5:36 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Mon, May 15, 2023 at 12:24:07PM -0700, Beau Belgrave wrote:
> > > >
> > > >   ret = pin_user_pages_remote(mm->mm, uaddr, 1, FOLL_WRITE | FOLL_NOFAULT,
> > > >                               &page, NULL, NULL);
> > >
> > > ... which will call pin_user_pages_remote() in RCU CS.
> > > This looks buggy, since pin_user_pages_remote() may schedule.
> > >
> >
> > If it's possible to schedule, I can change this to cache the probe
> > callbacks under RCU then drop it. However, when would
> > pin_user_pages_remote() schedule with FOLL_NOFAULT?
>
> Are you saying that passing FOLL_NOFAULT makes it work in atomic context?

Absolutely not.

It may not fault missing pages in, but that does *not* make it atomic.

That code depends on all the usual MM locking, and it does not work at
all in the same way that "pagefault_disable()" does, for example. That
will fail on any fault and never take locks, and is designed to work
in atomic contexts. Very different.

So no, don't think you can call pin_user_pages_remote() or any other
GUP function from atomic context.

We do have "get_user_page[s]_fast_only()" and that is the only version
of GUP that is actually lock-free.

Also, just FYI, those special gup_user*fast_only()" functions simply
will not work on some architectures at all.

               Linus