netdev - Re: seqlock lockdep false positives?

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Date:   Mon, 8 Mar 2021 21:42:08 +0100
From:   "Erhard F." <erhard_f@...lbox.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Jakub Kicinski <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: seqlock lockdep false positives?

On Mon, 8 Mar 2021 09:38:11 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> Indeed, that's one area where inlines are very much not equivalent to
> macros. Static variables in inline functions aren't exact, but they very
> much do not get to be one per invocation.
> 
> Something like the below ought to be the right fix I think.
> 
> diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
> index c6abb79501b3..e81856c0ba13 100644
> --- a/include/linux/u64_stats_sync.h
> +++ b/include/linux/u64_stats_sync.h
> @@ -115,12 +115,13 @@ static inline void u64_stats_inc(u64_stats_t *p)
>  }
>  #endif
>  
> +#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
> +#define u64_stats_init(syncp)	seqcount_init(&(syncp)->seq)
> +#else
>  static inline void u64_stats_init(struct u64_stats_sync *syncp)
>  {
> -#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
> -	seqcount_init(&syncp->seq);
> -#endif
>  }
> +#endif
>  
>  static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
>  {

Hi Peter!

I can confirm that your patch on top of 5.12-rc2 makes the lockdep splat disappear (Ahmeds' 1st patch not installed).