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From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: bigeasy@linutronix.de, peterz@infradead.org, tglx@linutronix.de,
	shung-hsi.yu@suse.com, linux-kernel@vger.kernel.org,
	Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH] seqlock,lockdep: Only check for preemption_disabled in non-rt
Date: Wed, 12 May 2021 11:47:13 +0200	[thread overview]
Message-ID: <YJukIUXFVdp1Fr7t@lx-t490> (raw)
In-Reply-To: <20210507233951.78950-1-dave@stgolabs.net>

On Fri, May 07, 2021, Davidlohr Bueso wrote:
> This silences the writer hitting this nonsensical warning on PREEMPT_RT.
>
> Reported-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> ---
>  include/linux/seqlock.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index f61e34fbaaea..c8f9253f1a2f 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -268,7 +268,9 @@ static inline bool __seqprop_preemptible(const seqcount_t *s)
>
>  static inline void __seqprop_assert(const seqcount_t *s)
>  {
> +#ifndef CONFIG_PREEMPT_RT
>  	lockdep_assert_preemption_disabled();
> +#endif
>  }
>

Nope, it is more complicated than that.

In general, for RT, seqcount_LOCKNAME_t variants should be used instead
of plain seqcount_t, as they can be safely used while preemption is
enabled on the write side.

For plain seqcount_t (which __seqprop_assert() is about), preemption
must be disabled, even for PREEMPT_RT. So the patch above is invalid.

Now, there are still some call sites in the kernel which needs
conversion obviously. I have a large patch series in queue which convert
a number of remaining networking call sites (the changes are locking
algorithm changes, not just direct substitution).

Good luck,

--
Ahmed S. Darwish
Linutronix GmbH

      parent reply	other threads:[~2021-05-12  9:47 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-07 23:39 Davidlohr Bueso
2021-05-07 23:47 ` [PATCH v2] " Davidlohr Bueso
2021-05-12  8:44   ` Peter Zijlstra
2021-05-13 19:53     ` Davidlohr Bueso
2021-05-14  4:55       ` Ahmed S. Darwish
2021-05-12  9:47 ` Ahmed S. Darwish [this message]

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