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

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Message-ID: <20200525112521.GD317569@hirez.programming.kicks-ass.net>
Date:   Mon, 25 May 2020 13:25:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrii Nakryiko <andriin@...com>
Cc:     paulmck@...nel.org, 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 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?