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Date:   Sat, 2 Sep 2023 08:59:46 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Dave Hansen <dave.hansen@linux.intel.com>
cc:     Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>,
        Bagas Sanjaya <bagasdotme@gmail.com>,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Gregory Greenman <gregory.greenman@intel.com>,
        Ben Greear <greearb@candelatech.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Networking <netdev@vger.kernel.org>,
        Linux Wireless <linux-wireless@vger.kernel.org>,
        Linux RCU <rcu@vger.kernel.org>
Subject: Re: Fwd: RCU indicates stalls with iwlwifi, causing boot failures
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References: <c1caa7c1-b2c6-aac5-54ab-8bcc6e139ca8@gmail.com> <c3f9b35c-087d-0e34-c251-e249f2c058d3@candelatech.com> <f0f6a6ec-e968-a91c-dc46-357566d8811@google.com>
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On Fri, 1 Sep 2023, Hugh Dickins wrote:

> Hi Dave,
> 
> On Fri, 1 Sep 2023, Ben Greear wrote:
> > On 9/1/23 5:29 PM, Bagas Sanjaya wrote:
> > > Hi,
> > > 
> > > I notice a bug report on Bugzilla [1]. Quoting from it:
> > 
> > Try booting with pcie=noaer ?
> > 
> > That fixes only known iwlwifi bug we have found in 6.5, but we are also using
> > mostly
> > backports iwlwifi driver...
> > 
> > Thanks,
> > Ben
> > 
> > > 
> > >> I'm seeing RCU warnings in Linus's current tree (like
> > >> 87dfd85c38923acd9517e8df4afc908565df0961) that come from RCU:
> > >>
> > >> WARNING: CPU: 0 PID: 0 at kernel/rcu/tree_exp.h:787
> > >> rcu_exp_handler+0x35/0xe0
> > >>
> > >> But they *ONLY* occur on a system with a newer iwlwifi device:
> > >>
> > >> aa:00.0 Network controller: Intel Corporation Wi-Fi 6 AX210/AX211/AX411
> > >> 160MHz (rev 1a)
> > >>
> > >> and never in a VM or on an older device (like an 8260).  During a bisect
> > >> the only seem to occur with the "83" version of the firmware.
> > >>
> > >> iwlwifi 0000:aa:00.0: loaded firmware version 83.e8f84e98.0
> > >> ty-a0-gf-a0-83.ucode op_mode iwlmvm
> > >>
> > >> The first warning gets spit out within a millisecond of the last printk()
> > >> from the iwlwifi driver.  They eventually result in a big spew of RCU
> > >> messages like this:
> > >>
> > >> [   27.124796] rcu: INFO: rcu_preempt detected expedited stalls on
> > >> CPUs/tasks: { 0-...D } 125 jiffies s: 193 root: 0x1/.
> > >> [   27.126466] rcu: blocking rcu_node structures (internal RCU debug):
> > >> [   27.128114] Sending NMI from CPU 3 to CPUs 0:
> > >> [   27.128122] NMI backtrace for cpu 0 skipped: idling at
> > >> intel_idle+0x5f/0xb0
> > >> [   27.159757] loop30: detected capacity change from 0 to 8
> > >> [   27.204967] rcu: INFO: rcu_preempt detected expedited stalls on
> > >> CPUs/tasks: { 0-...D } 145 jiffies s: 193 root: 0x1/.
> > >> [   27.206353] rcu: blocking rcu_node structures (internal RCU debug):
> > >> [   27.207751] Sending NMI from CPU 3 to CPUs 0:
> > >> [   27.207825] NMI backtrace for cpu 0 skipped: idling at
> > >> intel_idle+0x5f/0xb0
> > >>
> > >> I usually see them at boot.  In that case, they usually hang the system and
> > >> keep it from booting.  I've also encountered them at reboots and also seen
> > >> them *not* be fatal at boot.  I suspect it has to do with which CPU gets
> > >> wedged.
> > > 
> > > See Bugzilla for the full thread and attached full dmesg output.
> > > 
> > > Thanks.
> > > 
> > > [1]: https://bugzilla.kernel.org/show_bug.cgi?id=217856
> 
> I just took a look at your dmesg in bugzilla: I see lots of page tables
> dumped, including "ESPfix Area", and think you're hitting my screwup: see
> 
> https://lore.kernel.org/linux-mm/CABXGCsNi8Tiv5zUPNXr6UJw6qV1VdaBEfGqEAMkkXE3QPvZuAQ@mail.gmail.com/
> 
> Please give the patch from the end of that thread a try:

Mikhail confirmed it for his case, and Linus already took it into his tree:
ee40d543e97d mm/pagewalk: fix bootstopping regression from extra pte_unmap()

But I couldn't see the WARN_ON_ONCE I was expecting from __rcu_read_unlock()
in your dmesg - ah, but it is only when CONFIG_PROVE_LOCKING is enabled.

Hugh