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@xxxxxxxxxxxxx> 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@xxxxxxxxxx/ 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.
