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Date:   Fri, 6 Oct 2023 11:20:56 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc:     Naresh Kamboju <naresh.kamboju@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, patches@lists.linux.dev,
        linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        akpm@linux-foundation.org, linux@roeck-us.net, shuah@kernel.org,
        patches@kernelci.org, lkft-triage@lists.linaro.org, pavel@denx.de,
        jonathanh@nvidia.com, f.fainelli@gmail.com,
        sudipm.mukherjee@gmail.com, srw@sladewatkins.net, rwarsow@gmx.de,
        conor@kernel.org, Chengming Zhou <zhouchengming@bytedance.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ovidiu Panait <ovidiu.panait@windriver.com>,
        Ingo Molnar <mingo@kernel.org>, rcu <rcu@vger.kernel.org>
Subject: Re: [PATCH 5.15 000/183] 5.15.134-rc1 review
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On Fri, Oct 06, 2023 at 01:57:14PM -0400, Liam R. Howlett wrote:
> * Paul E. McKenney <paulmck@kernel.org> [231006 12:47]:
> > On Fri, Oct 06, 2023 at 12:20:38PM -0400, Liam R. Howlett wrote:
> > > * Naresh Kamboju <naresh.kamboju@linaro.org> [231005 13:49]:
> > > > On Wed, 4 Oct 2023 at 23:33, Greg Kroah-Hartman
> > > > <gregkh@linuxfoundation.org> wrote:
> > > > >
> > > > > This is the start of the stable review cycle for the 5.15.134 release.
> > > > > There are 183 patches in this series, all will be posted as a response
> > > > > to this one.  If anyone has any issues with these being applied, please
> > > > > let me know.
> > > > >
> > > > > Responses should be made by Fri, 06 Oct 2023 17:51:12 +0000.
> > > > > Anything received after that time might be too late.
> > > > >
> > > > > The whole patch series can be found in one patch at:
> > > > >         https://www.kernel.org/pub/linux/kernel/v5.x/stable-review/patch-5.15.134-rc1.gz
> > > > > or in the git tree and branch at:
> > > > >         git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git linux-5.15.y
> > > > > and the diffstat can be found below.
> > > > >
> > > > > thanks,
> > > > >
> > > > > greg k-h
> > > > 
> > > > Results from Linaro’s test farm.
> > > > Regressions on x86.
> > > > 
> > > > Following kernel warning noticed on x86 while booting stable-rc 5.15.134-rc1
> > > > with selftest merge config built kernel.
> > > > 
> > > > Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
> > > > 
> > > > Anyone noticed this kernel warning ?
> > > > 
> > > > This is always reproducible while booting x86 with a given config.
> > > 
> > > >From that config:
> > > #
> > > # RCU Subsystem
> > > #
> > > CONFIG_TREE_RCU=y
> > > # CONFIG_RCU_EXPERT is not set
> > > CONFIG_SRCU=y
> > > CONFIG_TREE_SRCU=y
> > > CONFIG_TASKS_RCU_GENERIC=y
> > > CONFIG_TASKS_RUDE_RCU=y
> > > CONFIG_TASKS_TRACE_RCU=y
> > > CONFIG_RCU_STALL_COMMON=y
> > > CONFIG_RCU_NEED_SEGCBLIST=y
> > > # end of RCU Subsystem    
> > > 
> > > #
> > > # RCU Debugging
> > > #
> > > CONFIG_PROVE_RCU=y
> > > # CONFIG_RCU_SCALE_TEST is not set
> > > # CONFIG_RCU_TORTURE_TEST is not set
> > > # CONFIG_RCU_REF_SCALE_TEST is not set
> > > CONFIG_RCU_CPU_STALL_TIMEOUT=21
> > > CONFIG_RCU_TRACE=y
> > > # CONFIG_RCU_EQS_DEBUG is not set
> > > # end of RCU Debugging
> > > 
> > > 
> > > > 
> > > > x86 boot log:
> > > > -----
> > > > [    0.000000] Linux version 5.15.134-rc1 (tuxmake@tuxmake)
> > > > (x86_64-linux-gnu-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils
> > > > for Debian) 2.40) #1 SMP @1696443178
> > > > ...
> > > > [    1.480701] ------------[ cut here ]------------
> > > > [    1.481296] WARNING: CPU: 0 PID: 13 at kernel/rcu/tasks.h:958
> > > > trc_inspect_reader+0x80/0xb0
> > > > [    1.481296] Modules linked in:
> > > > [    1.481296] CPU: 0 PID: 13 Comm: rcu_tasks_trace Not tainted 5.15.134-rc1 #1
> > > > [    1.481296] Hardware name: Supermicro SYS-5019S-ML/X11SSH-F, BIOS
> > > > 2.5 11/26/2020
> > > > [    1.481296] RIP: 0010:trc_inspect_reader+0x80/0xb0
> > > 
> > > This function has changed a lot, including the dropping of this
> > > WARN_ON_ONCE().  The warning was replaced in 897ba84dc5aa ("rcu-tasks:
> > > Handle idle tasks for recently offlined CPUs") with something that looks
> > > equivalent so I'm not sure why it would not trigger in newer revisions.
> > > 
> > > Obviously the behaviour I changed was the test for the task being idle.
> > > I am not sure how best to short-circuit that test from happening during
> > > boot as I am not familiar with the RCU code.
> > 
> > The usual test for RCU's notion of early boot being completed is
> > (rcu_scheduler_active != RCU_SCHEDULER_INIT).
> > 
> > Except that "ofl" should always be false that early in boot, at least
> > in mainline.
> 
> Is this still true in the final version of the patch where we set the
> boot task as !idle until just before the early boot is finished?  I
> wouldn't think of this as 'early in boot' anymore as much as the entire
> kernel setup.  Maybe we need to shorten the time we stay in !idle mode
> for earlier kernels?

In mainline, the ofl variable is defined as cpu_is_offline(cpu), and
during boot, the boot CPU is guaranteed to be online.  (As opposed to
the boot CPU's idle-task state.)

> How frequent is this function called?  We could check something for
> early boot... or track down where the cpu is put online and restore idle
> before that happens?

Once per RCU Tasks Trace grace period per reader seen to be blocking
that grace period.  Its performance is as issue, but not to anywhere
near the same extent as (say) rcu_read_lock_trace().

> > > It's also worth noting that the bug this fixes wasn't exposed until the
> > > maple tree (added in v6.1) was used for the IRQ descriptors (added in
> > > v6.5).
> > 
> > Lots of latent bugs, to be sure, even with rcutorture.  :-/
> 
> The Right Thing is to fix the bug all the way back to the introduction,
> but what fallout makes the backport less desirable than living with the
> unexposed bug?

You are quite right that it is possible for the risk of a backport to
exceed the risk of the original bug.

I defer to Joel (CCed) on how best to resolve this in -stable.

							Thanx, Paul