Re: [PATCH v2 00/12] uprobes: add batched register/unregister APIs and per-CPU RW semaphore

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 02, 2024 at 04:56:53PM -0700, Paul E. McKenney wrote:

> > Paul, isn't this the RCU flavour you created to deal with
> > !rcu_is_watching()? The flavour that never should have been created in
> > favour of just cleaning up the mess instead of making more.
> 
> My guess is that you are instead thinking of RCU Tasks Rude, which can
> be eliminated once all architectures get their entry/exit/deep-idle
> functions either inlined or marked noinstr.

Would it make sense to disable it for those architectures that have
already done this work?

> > > I will
> > > ultimately use it anyway to avoid uprobe taking unnecessary refcount
> > > and to protect uprobe->consumers iteration and uc->handler() calls,
> > > which could be sleepable, so would need rcu_read_lock_trace().
> > 
> > I don't think you need trace-rcu for that. SRCU would do nicely I think.
> 
> From a functional viewpoint, agreed.
> 
> However, in the past, the memory-barrier and array-indexing overhead
> of SRCU has made it a no-go for lightweight probes into fastpath code.
> And these cases were what motivated RCU Tasks Trace (as opposed to RCU
> Tasks Rude).

I'm thinking we're growing too many RCU flavours again :/ I suppose I'll
have to go read up on rcu/tasks.* and see what's what.

> The other rule for RCU Tasks Trace is that although readers are permitted
> to block, this blocking can be for no longer than a major page fault.
> If you need longer-term blocking, then you should instead use SRCU.

I think this would render it unsuitable for uprobes. The whole point of
having a sleepable handler is to be able to take faults.