linux-kernel - Re: [PATCH RT 1/5] net: Properly annotate the try-lock for the seqlock

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Date:   Sat, 14 Nov 2020 20:00:07 +0100
From:   Mike Galbraith <efault@....de>
To:     Tom Zanussi <zanussi@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        John Kacur <jkacur@...hat.com>, Daniel Wagner <wagi@...om.org>,
        "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>,
        stable-rt@...r.kernel.org
Subject: Re: [PATCH RT 1/5] net: Properly annotate the try-lock for the
 seqlock

On Fri, 2020-11-13 at 14:14 -0600, Tom Zanussi wrote:
>
> This patch seems to fix it for me:

If there was any discussion about this patch, I missed it.

> From 4855377d0cb34b1b67a5c6d84cc8609c9da0bc3e Mon Sep 17 00:00:00 2001
> Message-Id: <4855377d0cb34b1b67a5c6d84cc8609c9da0bc3e.1605297603.git.zanussi@...nel.org>
> From: Tom Zanussi <zanussi@...nel.org>
> Date: Fri, 13 Nov 2020 13:04:15 -0600
> Subject: [PATCH] net: Add missing __raw_write_seqcount_end() and
>  seqcount_release()
>
> The patch ('net: Properly annotate the try-lock for the seqlock") adds
> __raw_write_seqcount_begin() in qdisc_run_begin() but omits the
> corresponding __raw_write_seqcount_end() and seqcount_release() in
> qdisc_run_end().
>
> Add it unconditionally, since qdisc_run_end() is never called unless
> qdisc_run_begin() succeeds, and if it succeeds,
> __raw_write_seqcount_begin() seqcount_acquire() will have been called.
>
> Signed-off-by: Tom Zanussi <zanussi@...nel.org>
> ---
>  include/net/sch_generic.h | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index 112d2dca8b08..c5ccce4f8f62 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -192,7 +192,11 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
>  static inline void qdisc_run_end(struct Qdisc *qdisc)
>  {
>  #ifdef CONFIG_PREEMPT_RT
> +	seqcount_t *s = &qdisc->running.seqcount;
> +
>  	write_sequnlock(&qdisc->running);
> +	__raw_write_seqcount_end(s);
> +	seqcount_release(&s->dep_map, 1, _RET_IP_);
>  #else
>  	write_seqcount_end(&qdisc->running);
>  #endif

__raw_write_seqcount_end() is an integral part of write_sequnlock(),
but we do seem to be missing a seqcount_release() in 5.4-rt.

	-Mike