linux-kernel - Re: Some -serious- BPF-related litmus tests

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Date:   Mon, 25 May 2020 08:47:30 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrii Nakryiko <andriin@...com>,
        Alan Stern <stern@...land.harvard.edu>, parri.andrea@...il.com,
        will@...nel.org, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com, dlustig@...dia.com, joel@...lfernandes.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>
Subject: Re: Some -serious- BPF-related litmus tests

On Mon, May 25, 2020 at 01:25:21PM +0200, Peter Zijlstra wrote:
> On Fri, May 22, 2020 at 12:38:21PM -0700, Andrii Nakryiko wrote:
> > On 5/22/20 10:43 AM, Paul E. McKenney wrote:
> > > On Fri, May 22, 2020 at 10:32:01AM -0400, Alan Stern wrote:
> 
> > > > Also, what use is a spinlock that is accessed in only one thread?
> > > 
> > > Multiple writers synchronize via the spinlock in this case.  I am
> > > guessing that his larger 16-hour test contended this spinlock.
> > 
> > Yes, spinlock is for coordinating multiple producers. 2p1c cases (bounded
> > and unbounded) rely on this already. 1p1c cases are sort of subsets (but
> > very fast to verify) checking only consumer/producer interaction.
> 
> Does that spinlock imply that we can now never fix that atrocious
> bpf_prog_active trainwreck ?
> 
> How does that spinlock not trigger the USED <- IN-NMI lockdep check:
> 
>   f6f48e180404 ("lockdep: Teach lockdep about "USED" <- "IN-NMI" inversions")
> 
> ?
> 
> That is; how can you use a spinlock on the producer side at all?

So even trylock is now forbidden in NMI handlers?  If so, why?

							Thanx, Paul