linux-kernel - Re: Tasks RCU, ftrace, and trampolines (was: Re: [PATCH 00/30] PREEMPT

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Date: Tue, 19 Mar 2024 16:33:18 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Ankur Arora <ankur.a.arora@...cle.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	peterz@...radead.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
	dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	willy@...radead.org, mgorman@...e.de, jpoimboe@...nel.org,
	jgross@...e.com, andrew.cooper3@...rix.com, bristot@...nel.org,
	mathieu.desnoyers@...icios.com, glaubitz@...sik.fu-berlin.de,
	anton.ivanov@...bridgegreys.com, mattst88@...il.com,
	krypton@...ich-teichert.org, David.Laight@...lab.com,
	richard@....at, jon.grimm@....com, bharata@....com,
	boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: Tasks RCU, ftrace, and trampolines (was: Re: [PATCH 00/30]
 PREEMPT_AUTO: support lazy rescheduling)

On Tue, Mar 19, 2024 at 11:45:15AM +0000, Mark Rutland wrote:
> Hi Paul,
> 
> On Fri, Mar 01, 2024 at 05:16:33PM -0800, Paul E. McKenney wrote:
> > The networking NAPI code ends up needing special help to avoid starving
> > Tasks RCU grace periods [1].  I am therefore revisiting trying to make
> > Tasks RCU directly detect trampoline usage, but without quite as much
> > need to identify specific trampolines...
> > 
> > I am putting this information in a Google document for future
> > reference [2].
> > 
> > Thoughts?
> 
> Sorry for the long delay! I've been looking into this general area over the
> last couple of weeks due to the latent bugs I mentioned in:
> 
>   https://lore.kernel.org/lkml/Zenx_Q0UiwMbSAdP@FVFF77S0Q05N/
> 
> I was somewhat hoping that staring at the code for long enough would result in
> an ephinany (and a nice simple-to-backport solution for the latent issues), but
> so far that has eluded me.
> 
> I believe some of those cases will need to use synchronize_rcu_tasks() and we
> might be able to make some structural changes to minimize the number of times
> we'd need to synchronize (e.g. having static ftrace call ops->func from the ops
> pointer, so we can switch ops+func atomically), but those look pretty invasive
> so far.
> 
> I haven't been able to come up with "a precise and completely reliable way to
> determine whether the current preemption occurred within a trampoline". Since
> preemption might occur within a trampoline's callee that eventually returns
> back to the trampoline, I believe that'll either depend on having a reliable
> stacktrace or requiring the trampoline to dynamically register/unregister
> somewhere around calling other functions. That, and we do also care about those
> callees themselves, and it's not just about the trampolines...
> 
> On arm64, we kinda have "permanent trampolines", as our
> DYNAMIC_FTRACE_WILL_CALL_OPS implementation uses a common trampoline. However,
> that will tail-call direct functions (and those could also be directly called
> from ftrace callsites), so we don't have a good way of handling those without a
> change to the direct func calling convention.
> 
> I assume that permanent trampolines wouldn't be an option on architectures
> where trampolines are a spectre mitigation.

Thank you for checking!  I placed a pointer to this email in the document
and updated the relevant sections accordingly.

> Mark.
> 
> > 								Thanx, Paul
> > 
> > [1] https://lore.kernel.org/all/Zd4DXTyCf17lcTfq@debian.debian/
> > [2] https://docs.google.com/document/d/1kZY6AX-AHRIyYQsvUX6WJxS1LsDK4JA2CHuBnpkrR_U/edit?usp=sharing