Re: [PATCH v2 tip/perf/core 1/2] uprobes: allow put_uprobe() from non-sleepable softirq context

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Oct 18, 2024 at 11:22:00AM -0700, Andrii Nakryiko wrote:
> On Fri, Oct 18, 2024 at 1:26 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Mon, Oct 07, 2024 at 05:25:55PM -0700, Andrii Nakryiko wrote:
> > > Currently put_uprobe() might trigger mutex_lock()/mutex_unlock(), which
> > > makes it unsuitable to be called from more restricted context like softirq.
> >
> > This is delayed_uprobe_lock, right?
> 
> Not just delated_uprobe_lock, there is also uprobes_treelock (I forgot
> to update the commit message to mention that). Oleg had concerns (see
> [0]) with that being taken from the timer thread, so I just moved all
> of the locking into deferred work callback.
> 
>   [0] https://lore.kernel.org/linux-trace-kernel/20240915144910.GA27726@redhat.com/

Right, but at least that's not a sleeping lock. He's right about it
needing to become a softirq-safe lock though. And yeah, unfortunate
that.

> > So can't we do something like so instead?
> 
> I'll need to look at this more thoroughly (and hopefully Oleg will get
> a chance as well), dropping lock from delayed_ref_ctr_inc() is a bit
> scary, but might be ok.

So I figured that update_ref_ctr() is already doing the
__update_ref_ctr() thing without holding the lock, so that lock really
is only there to manage the list.

And that list is super offensive... That really wants to be a per-mm
rb-tree or somesuch.

AFAICT the only reason it is a mutex, is because doing unbouded list
iteration under a spinlock is a really bad idea.

> But generally speaking, what's your concern with doing deferred work
> in put_uprobe()? It's not a hot path by any means, worst case we'll
> have maybe thousands of uprobes attached/detached.

Mostly I got offended by the level of crap in that code, and working
around crap instead of fixing crap just ain't right.