Re: WARNING: suspicious RCU usage - sdhci-pltfm: SDHCI platform and OF driver helper

[Ulf Hansson]

+ Saravanna, Rafael, Lina

On Mon, 31 Aug 2020 at 21:44, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Mon, Aug 31, 2020 at 12:02:31PM +0530, Naresh Kamboju wrote:
> > While booting linux mainline kernel on arm64 db410c this kernel warning
> > noticed.
> >
> > metadata:
> >   git branch: master
> >   git repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> >   git commit: f75aef392f869018f78cfedf3c320a6b3fcfda6b
> >   git describe: v5.9-rc3
> >   make_kernelversion: 5.9.0-rc3
> >   kernel-config:
> > http://snapshots.linaro.org/openembedded/lkft/lkft/sumo/dragonboard-410c/lkft/linux-mainline/2965/config
> >
> > Boot log,
> >
> > [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd030]
> > [    0.000000] Linux version 5.9.0-rc3 (oe-user@oe-host)
> > (aarch64-linaro-linux-gcc (GCC) 7.3.0, GNU ld (GNU Binutils)
> > 2.30.0.20180208) #1 SMP PREEMPT Mon Aug 31 00:23:15 UTC 2020
> > [    0.000000] Machine model: Qualcomm Technologies, Inc. APQ 8016 SBC
> > <>
> > [    5.299090] sdhci: Secure Digital Host Controller Interface driver
> > [    5.299140] sdhci: Copyright(c) Pierre Ossman
> > [    5.304313]
> > [    5.307771] Synopsys Designware Multimedia Card Interface Driver
> > [    5.308588] =============================
> > [    5.308593] WARNING: suspicious RCU usage
> > [    5.316628] sdhci-pltfm: SDHCI platform and OF driver helper
> > [    5.320052] 5.9.0-rc3 #1 Not tainted
> > [    5.320057] -----------------------------
> > [    5.320063] /usr/src/kernel/include/trace/events/lock.h:37
> > suspicious rcu_dereference_check() usage!
> > [    5.320068]
> > [    5.320068] other info that might help us debug this:
> > [    5.320068]
> > [    5.320074]
> > [    5.320074] rcu_scheduler_active = 2, debug_locks = 1
> > [    5.320078] RCU used illegally from extended quiescent state!
> > [    5.320084] no locks held by swapper/0/0.
> > [    5.320089]
> > [    5.320089] stack backtrace:
> > [    5.320098] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.9.0-rc3 #1
> > [    5.346354] sdhci_msm 7864900.sdhci: Got CD GPIO
> > [    5.346446] Hardware name: Qualcomm Technologies, Inc. APQ 8016 SBC (DT)
> > [    5.346452] Call trace:
> > [    5.346463]  dump_backtrace+0x0/0x1f8
> > [    5.346471]  show_stack+0x2c/0x38
> > [    5.346480]  dump_stack+0xec/0x15c
> > [    5.346490]  lockdep_rcu_suspicious+0xd4/0xf8
> > [    5.346499]  lock_acquire+0x3d0/0x440
> > [    5.346510]  _raw_spin_lock_irqsave+0x80/0xb0
> > [    5.413118]  __pm_runtime_suspend+0x34/0x1d0
> > [    5.417457]  psci_enter_domain_idle_state+0x4c/0xb0
> > [    5.421795]  cpuidle_enter_state+0xc8/0x610
> > [    5.426392]  cpuidle_enter+0x3c/0x50
> > [    5.430561]  call_cpuidle+0x44/0x80
> > [    5.434378]  do_idle+0x240/0x2a0
>
> RCU ignores CPUs in the idle loop, which means that you cannot use
> rcu_read_lock() from the idle loop without use of something like
> RCU_NONIDLE().  If this is due to event tracing, you should use the
> _rcuidle() variant of the event trace statement.

In the runtime suspend path, the runtime PM core calls
device_links_read_lock() - if the device in question has any links to
suppliers (to allow them to be suspended too).

device_links_read_lock() calls srcu_read_lock().

It turns out that the device in question (the CPU device that is
attached to genpd) didn't have any links before - but that seems to
have changed, due to the work done by Saravana (links become created
on a per resource basis, parsed from DT during boot).

>
> Note also that Peter Zijlstra (CCed) is working to shrink the portion
> of the idle loop that RCU ignores.  Not sure that it covers your
> case, but it is worth checking.

Thanks for letting me know. Let's see what Peter thinks about this then.

Apologize for my ignorance, but from a cpuidle point of view, what
does it mean using RCU_NONIDLE()? I guess we should avoid RCU_NONIDLE
on bigger code paths?

I could add RCU_NONIDLE for the calls to pm_runtime_put_sync_suspend()
and pm_runtime_get_sync() in psci_enter_domain_idle_state(). Perhaps
that's the easiest approach, at least to start with.

Or do you have any other ideas?

[...]

Kind regards
Uffe