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

[peterz@infradead.org]

On Tue, Sep 01, 2020 at 08:50:57AM +0200, Ulf Hansson wrote:
> On Tue, 1 Sep 2020 at 08:46, Ulf Hansson <ulf.hansson@linaro.org> wrote:
> > On Mon, 31 Aug 2020 at 21:44, Paul E. McKenney <paulmck@kernel.org> wrote:

> > > > [    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

> > > 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.

Right, so I think I 'caused' this by making the lock tracepoints
visible. That is, the error always existed, now we actually warn about
it.

> > 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?

So IMO trace_*_rcuidle() and RCU_NONIDLE() are bugs, they just mean we
got the ordering wrong and are papering over it. That said, that's been
the modus operandi for a while now, just make it shut up and don't think
about it :-/

That said; I pushed the rcu_idle_enter() about as deep as it goes into
generic code in commit 1098582a0f6c ("sched,idle,rcu: Push rcu_idle
deeper into the idle path")

I suppose the next step is pushing it into individual driver when
needed, something like the below perhaps. I realize the coupled idle
state stuff is more complicated that most, but it's also not an area
I've looked at in detail, so perhaps I've just made a bigger mess, but
it ought to give you enough to get going I think.

Rafael?

---
diff --git a/drivers/cpuidle/cpuidle-psci.c b/drivers/cpuidle/cpuidle-psci.c
index 74463841805f..617bbef316e6 100644
--- a/drivers/cpuidle/cpuidle-psci.c
+++ b/drivers/cpuidle/cpuidle-psci.c
@@ -49,6 +49,9 @@ static inline u32 psci_get_domain_state(void)
 
 static inline int psci_enter_state(int idx, u32 state)
 {
+	/*
+	 * XXX push rcu_idle_enter into the coupled code
+	 */
 	return CPU_PM_CPU_IDLE_ENTER_PARAM(psci_cpu_suspend_enter, idx, state);
 }
 
@@ -72,7 +75,9 @@ static int psci_enter_domain_idle_state(struct cpuidle_device *dev,
 	if (!state)
 		state = states[idx];
 
+	rcu_idle_enter();
 	ret = psci_cpu_suspend_enter(state) ? -1 : idx;
+	rcu_idle_exit();
 
 	pm_runtime_get_sync(pd_dev);
 
@@ -125,8 +130,13 @@ static int psci_enter_idle_state(struct cpuidle_device *dev,
 				struct cpuidle_driver *drv, int idx)
 {
 	u32 *state = __this_cpu_read(psci_cpuidle_data.psci_states);
+	int ret;
 
-	return psci_enter_state(idx, state[idx]);
+	rcu_idle_enter();
+	ret = psci_enter_state(idx, state[idx]);
+	rcu_idle_exit();
+
+	return ret;
 }
 
 static const struct of_device_id psci_idle_state_match[] = {
@@ -170,6 +180,7 @@ static int psci_dt_cpu_init_topology(struct cpuidle_driver *drv,
 	 * deeper states.
 	 */
 	drv->states[state_count - 1].enter = psci_enter_domain_idle_state;
+	drv->states[state_count - 1].flags = CPUIDLE_FLAG_RCU_IDLE;
 	psci_cpuidle_use_cpuhp = true;
 
 	return 0;
@@ -285,6 +296,7 @@ static int psci_idle_init_cpu(struct device *dev, int cpu)
 	 * state index 0.
 	 */
 	drv->states[0].enter = psci_enter_idle_state;
+	drv->states[0].flags = CPUIDLE_FLAG_RCU_IDLE;
 	drv->states[0].exit_latency = 1;
 	drv->states[0].target_residency = 1;
 	drv->states[0].power_usage = UINT_MAX;
diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index 04becd70cc41..3dbac3bb761b 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -239,9 +239,11 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
 	time_start = ns_to_ktime(local_clock());
 
 	stop_critical_timings();
-	rcu_idle_enter();
+	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
+		rcu_idle_enter();
 	entered_state = target_state->enter(dev, drv, index);
-	rcu_idle_exit();
+	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
+		rcu_idle_exit();
 	start_critical_timings();
 
 	sched_clock_idle_wakeup_event();
diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
index 75895e6363b8..47f686131a54 100644
--- a/include/linux/cpuidle.h
+++ b/include/linux/cpuidle.h
@@ -82,6 +82,7 @@ struct cpuidle_state {
 #define CPUIDLE_FLAG_UNUSABLE		BIT(3) /* avoid using this state */
 #define CPUIDLE_FLAG_OFF		BIT(4) /* disable this state by default */
 #define CPUIDLE_FLAG_TLB_FLUSHED	BIT(5) /* idle-state flushes TLBs */
+#define CPUIDLE_FLAG_RCU_IDLE		BIT(6) /* driver will do RCU-idle */
 
 struct cpuidle_device_kobj;
 struct cpuidle_state_kobj;