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

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Date:   Mon, 16 Nov 2020 18:19:58 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Mike Galbraith <efault@....de>
Cc:     Tom Zanussi <zanussi@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        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 2020-11-15 05:52:33 [+0100], Mike Galbraith wrote:
> On Sat, 2020-11-14 at 13:24 -0600, Tom Zanussi wrote:
> > On Sat, 2020-11-14 at 20:00 +0100, Mike Galbraith wrote:
> >
> > > __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.
> > >
> >
> > Yep, you're right, it's just the missing seqcount_release() - I'll
> > resubmit with just that.
> 
> Or just drop the backport, since it adds annotation, while the original
> was fixing existing annotation.
> 
> __raw_write_seqcount_begin() called in 5.4-rt try_write_seqlock() is
> not annotated, while write_seqcount_begin() called by the 5.9-rt
> version leads to the broken annotation that the original then fixed.

That is correct.
I was looking at the 5.4-RT series Steven posted and I was under the
impression that this patch was correctly missing in previous RT since I
even added the stable tag.
As Mike said, the previous RT implementation did not use seqlock
annotation, they used a spin-lock instead. So the "try_write_seqlock()"
did the try-lock annotation.
With the reworked seqcount implementation (v5.6-RT time frame) this was
solved differently (closer to what upstream does) and the now the
annotation was wrong and fixed.

Sorry for that.

> 	-Mike

Sebastian