linux-kernel - Re: [PATCH v2 1/2] maple_tree: Disable mas_wr_append() when other readers are possible

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Message-ID: <f9b0a88c-8a64-439f-a488-85d500c9f2aa@paulmck-laptop>
Date:   Tue, 12 Sep 2023 01:30:06 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org,
        Shanker Donthineni <sdonthineni@...dia.com>
Subject: Re: [PATCH v2 1/2] maple_tree: Disable mas_wr_append() when other
 readers are possible

On Tue, Sep 12, 2023 at 10:23:37AM +0200, Geert Uytterhoeven wrote:
> Hi Paul,
> 
> On Tue, Sep 12, 2023 at 10:14 AM Paul E. McKenney <paulmck@...nel.org> wrote:
> > On Mon, Sep 11, 2023 at 07:54:52PM -0400, Liam R. Howlett wrote:
> > > * Paul E. McKenney <paulmck@...nel.org> [230906 14:03]:
> > > > On Wed, Sep 06, 2023 at 01:29:54PM -0400, Liam R. Howlett wrote:
> > > > > * Paul E. McKenney <paulmck@...nel.org> [230906 13:24]:
> > > > > > On Wed, Sep 06, 2023 at 11:23:25AM -0400, Liam R. Howlett wrote:
> > > > > > > (Adding Paul & Shanker to Cc list.. please see below for why)
> > > > > > >
> > > > > > > Apologies on the late response, I was away and have been struggling to
> > > > > > > get a working PPC32 test environment.
> > > > > > >
> > > > > > > * Geert Uytterhoeven <geert@...ux-m68k.org> [230829 12:42]:
> > > > > > > >     Hi Liam,
> > > > > > > >
> > > > > > > > On Fri, 18 Aug 2023, Liam R. Howlett wrote:
> > > > > > > > > The current implementation of append may cause duplicate data and/or
> > > > > > > > > incorrect ranges to be returned to a reader during an update.  Although
> > > > > > > > > this has not been reported or seen, disable the append write operation
> > > > > > > > > while the tree is in rcu mode out of an abundance of caution.
> > > > > > >
> > > > > > > ...
> > > > > > > > >
> > >
> > > ...
> > >
> > > > > > > > RCU-related configs:
> > > > > > > >
> > > > > > > >     $ grep RCU .config
> > > > > > > >     # RCU Subsystem
> > > > > > > >     CONFIG_TINY_RCU=y
> >
> > I must have been asleep last time I looked at this.  I was looking at
> > Tree RCU.  Please accept my apologies for my lapse.  :-/
> >
> > However, Tiny RCU's call_rcu() also avoids enabling IRQs, so I would
> > have said the same thing, albeit after looking at a lot less RCU code.
> >
> > TL;DR:
> >
> > 1.      Try making the __setup_irq() function's call to mutex_lock()
> >         instead be as follows:
> >
> >         if (!mutex_trylock(&desc->request_mutex))
> >                 mutex_lock(&desc->request_mutex);
> >
> >         This might fail if __setup_irq() has other dependencies on a
> >         fully operational scheduler.
> >
> > 2.      Move that ppc32 call to __setup_irq() much later, most definitely
> >         after interrupts have been enabled and the scheduler is fully
> >         operational.  Invoking mutex_lock() before that time is not a
> >         good idea.  ;-)
> 
> There is no call to __setup_irq() from arch/powerpc/?

Glad it is not just me, given that I didn't see a direct call, either.  So
later in this email, I asked Liam to put a WARN_ON_ONCE(irqs_disabled())
just before that mutex_lock() in __setup_irq().

Either way, invoking mutex_lock() early in boot before interrupts have
been enabled is a bad idea.  ;-)

> Note that there are (possibly different) issues seen on ppc32 and on arm32
> (Renesas RZ/A in particular, but not on other Renesas ARM systems).
> 
> I saw an issue on arm32 with cfeb6ae8bcb96ccf, but not with cfeb6ae8bcb96ccf^.
> Other people saw an issue on ppc32 with both cfeb6ae8bcb96ccf and
> cfeb6ae8bcb96ccf^.

I look forward to hearing what is the issue in both cases.

							Thanx, Paul

> Thanks!
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> -- 
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds