Re: RCU used on incoming CPU before rcu_cpu_starting() called

Paul E. McKenney
2017-03-08T23:43:48Z

On Wed, Mar 08, 2017 at 02:16:56PM -0800, Paul E. McKenney wrote:
> Hello!
>
> I am seeing the following splat in rcutorture testing of v4.11-rc1:
>
> [ 30.694013] =============================
> [ 30.694013] WARNING: suspicious RCU usage
> [ 30.694013] 4.11.0-rc1+ #1 Not tainted
> [ 30.694013] -----------------------------
> [ 30.694013] /home/git/linux-2.6-tip/kernel/workqueue.c:712 sched RCU or wq_pool_mutex should be held!
> [ 30.694013]
> [ 30.694013] other info that might help us debug this:
> [ 30.694013]
> [ 30.694013]
> [ 30.694013] RCU used illegally from offline CPU!
> [ 30.694013] rcu_scheduler_active = 2, debug_locks = 0
> [ 30.694013] no locks held by swapper/1/0.
> [ 30.694013]
> [ 30.694013] stack backtrace:
> [ 30.694013] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.11.0-rc1+ #1
> [ 30.694013] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> [ 30.694013] Call Trace:
> [ 30.694013] dump_stack+0x67/0x99
> [ 30.694013] lockdep_rcu_suspicious+0xe7/0x120
> [ 30.694013] get_work_pool+0x82/0x90
> [ 30.694013] __queue_work+0x70/0x5f0
> [ 30.694013] queue_work_on+0x33/0x70
> [ 30.694013] clear_sched_clock_stable+0x33/0x40
> [ 30.694013] early_init_intel+0xe7/0x2f0
> [ 30.694013] init_intel+0x11/0x350
> [ 30.694013] identify_cpu+0x344/0x5a0
> [ 30.694013] identify_secondary_cpu+0x18/0x80
> [ 30.694013] smp_store_cpu_info+0x39/0x40
> [ 30.694013] start_secondary+0x4e/0x100
> [ 30.694013] start_cpu+0x14/0x14
>
> Here is the relevant code from x86's smp_callin():
>
> /*
> * Save our processor parameters. Note: this information
> * is needed for clock calibration.
> */
> smp_store_cpu_info(cpuid);
>
> /*
> * Get our bogomips.
> * Update loops_per_jiffy in cpu_data. Previous call to
> * smp_store_cpu_info() stored a value that is close but not as
> * accurate as the value just calculated.
> */
> calibrate_delay();
> cpu_data(cpuid).loops_per_jiffy = loops_per_jiffy;
> pr_debug("Stack at about %p\n", &cpuid);
>
> /*
> * This must be done before setting cpu_online_mask
> * or calling notify_cpu_starting.
> */
> set_cpu_sibling_map(raw_smp_processor_id());
> wmb();
>
> notify_cpu_starting(cpuid);
>
> The problem is that smp_store_cpu_info() indirectly invokes
> schedule_work(), which wants to use RCU. But RCU isn't informed
> of the incoming CPU until the call to notify_cpu_starting(), which
> causes lockdep to complain bitterly about the use of RCU by the
> premature call to schedule_work().
>
> I considered just moving the notify_cpu_starting() earlier in the
> sequence, but the comments make it seem like this would not be
> a wise choice.
>
> Any suggestions?

And I should hasten to mention my default approach, which would be
to provide a Kconfig option that causes notify_cpu_starting() to
skip calling rcu_cpu_starting(), and to add a call to this function
in x86's smp_callin() just before the call to smp_store_cpu_info().
But this seemed a bit crude, so I figured I should check with you
guys before taking that approach.

But what is the fun in that? Please see below for the corresponding
lightly tested patch. ;-)

Thanx, Paul

------------------------------------------------------------------------

commit aa805dad9ae66cc4f5106d004b6bb5102fd84434
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Wed Mar 8 15:32:29 2017 -0800

cpu: Move RCU earlier in x86 CPU-online procedure

Running rcutorture on v4.11-rc1 results in the following splat on x86
on kernels built with both CPU hotplug and lockdep:

[ 30.694013] =============================
[ 30.694013] WARNING: suspicious RCU usage
[ 30.694013] 4.11.0-rc1+ #1 Not tainted
[ 30.694013] -----------------------------
[ 30.694013] /home/git/linux-2.6-tip/kernel/workqueue.c:712 sched RCU or wq_pool_mutex should be held!
[ 30.694013]
[ 30.694013] other info that might help us debug this:
[ 30.694013]
[ 30.694013]
[ 30.694013] RCU used illegally from offline CPU!
[ 30.694013] rcu_scheduler_active = 2, debug_locks = 0
[ 30.694013] no locks held by swapper/1/0.
[ 30.694013]
[ 30.694013] stack backtrace:
[ 30.694013] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.11.0-rc1+ #1
[ 30.694013] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[ 30.694013] Call Trace:
[ 30.694013] dump_stack+0x67/0x99
[ 30.694013] lockdep_rcu_suspicious+0xe7/0x120
[ 30.694013] get_work_pool+0x82/0x90
[ 30.694013] __queue_work+0x70/0x5f0
[ 30.694013] queue_work_on+0x33/0x70
[ 30.694013] clear_sched_clock_stable+0x33/0x40
[ 30.694013] early_init_intel+0xe7/0x2f0
[ 30.694013] init_intel+0x11/0x350
[ 30.694013] identify_cpu+0x344/0x5a0
[ 30.694013] identify_secondary_cpu+0x18/0x80
[ 30.694013] smp_store_cpu_info+0x39/0x40
[ 30.694013] start_secondary+0x4e/0x100
[ 30.694013] start_cpu+0x14/0x14

This is caused by the fact that smp_store_cpu_info() indirectly invokes
schedule_work(), which wants to use RCU. But RCU isn't informed of the
incoming CPU until the later call to notify_cpu_starting(), which causes
lockdep to complain bitterly about the use of RCU by the premature call
to schedule_work(). It is said to be unwise to move the call to
notify_cpu_starting() to precede that to smp_store_cpu_info(), so this
commit adds a ARCH_RCU_ONLINE_EARLY Kconfig option that is selected by
the x86 architecture. This option suppresses the call to rcu_cpu_starting()
from notify_cpu_starting(), and a call to rcu_cpu_starting() is added
preceding the call to smp_store_cpu_info().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index cc98d5a294ee..bb42b76ec2dd 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -63,6 +63,7 @@ config X86
select ARCH_MIGHT_HAVE_ACPI_PDC if ACPI
select ARCH_MIGHT_HAVE_PC_PARPORT
select ARCH_MIGHT_HAVE_PC_SERIO
+ select ARCH_RCU_ONLINE_EARLY
select ARCH_SUPPORTS_ATOMIC_RMW
select ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT
select ARCH_SUPPORTS_NUMA_BALANCING if X86_64
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index bd1f1ad35284..ca7386d04194 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -188,6 +188,12 @@ static void smp_callin(void)
apic_ap_setup();

/*
+ * Enable RCU readers on this CPU, needed for workqueues,
+ * which are used in smp_store_cpu_info().
+ */
+ rcu_cpu_starting(cpuid);
+
+ /*
* Save our processor parameters. Note: this information
* is needed for clock calibration.
*/
diff --git a/init/Kconfig b/init/Kconfig
index c859c993c26f..31c2d5e08f16 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -543,6 +543,9 @@ config RCU_STALL_COMMON
the tiny variants to disable RCU CPU stall warnings, while
making these warnings mandatory for the tree variants.

+config ARCH_RCU_ONLINE_EARLY
+ bool
+
config CONTEXT_TRACKING
bool

diff --git a/kernel/cpu.c b/kernel/cpu.c
index f7c063239fa5..9d98f2785fa0 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -851,7 +851,8 @@ void notify_cpu_starting(unsigned int cpu)
struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
enum cpuhp_state target = min((int)st->target, CPUHP_AP_ONLINE);

- rcu_cpu_starting(cpu); /* Enables RCU usage on this CPU. */
+ if (!IS_ENABLED(CONFIG_ARCH_RCU_ONLINE_EARLY))
+ rcu_cpu_starting(cpu); /* Enables RCU usage on this CPU. */
while (st->state < target) {
st->state++;
cpuhp_invoke_callback(cpu, st->state, true, NULL);