[PATCH AUTOSEL 5.7 145/274] sched/core: Fix illegal RCU from offline CPUs

Sasha Levin
2020-06-09T00:55:38Z

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit bf2c59fce4074e55d622089b34be3a6bc95484fb ]

In the CPU-offline process, it calls mmdrop() after idle entry and the
subsequent call to cpuhp_report_idle_dead(). Once execution passes the
call to rcu_report_dead(), RCU is ignoring the CPU, which results in
lockdep complaining when mmdrop() uses RCU from either memcg or
debugobjects below.

Fix it by cleaning up the active_mm state from BP instead. Every arch
which has CONFIG_HOTPLUG_CPU should have already called idle_task_exit()
from AP. The only exception is parisc because it switches them to
&init_mm unconditionally (see smp_boot_one_cpu() and smp_cpu_init()),
but the patch will still work there because it calls mmgrab(&init_mm) in
smp_cpu_init() and then should call mmdrop(&init_mm) in finish_cpu().

WARNING: suspicious RCU usage
-----------------------------
kernel/workqueue.c:710 RCU or wq_pool_mutex should be held!

other info that might help us debug this:

RCU used illegally from offline CPU!
Call Trace:
dump_stack+0xf4/0x164 (unreliable)
lockdep_rcu_suspicious+0x140/0x164
get_work_pool+0x110/0x150
__queue_work+0x1bc/0xca0
queue_work_on+0x114/0x120
css_release+0x9c/0xc0
percpu_ref_put_many+0x204/0x230
free_pcp_prepare+0x264/0x570
free_unref_page+0x38/0xf0
__mmdrop+0x21c/0x2c0
idle_task_exit+0x170/0x1b0
pnv_smp_cpu_kill_self+0x38/0x2e0
cpu_die+0x48/0x64
arch_cpu_idle_dead+0x30/0x50
do_idle+0x2f4/0x470
cpu_startup_entry+0x38/0x40
start_secondary+0x7a8/0xa80
start_secondary_resume+0x10/0x14

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Link: https://lkml.kernel.org/r/20200401214033.8448-1-cai@lca.pw
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/powerpc/platforms/powernv/smp.c | 1 -
include/linux/sched/mm.h | 2 ++
kernel/cpu.c | 18 +++++++++++++++++-
kernel/sched/core.c | 5 +++--
4 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/platforms/powernv/smp.c b/arch/powerpc/platforms/powernv/smp.c
index 13e251699346..b2ba3e95bda7 100644
--- a/arch/powerpc/platforms/powernv/smp.c
+++ b/arch/powerpc/platforms/powernv/smp.c
@@ -167,7 +167,6 @@ static void pnv_smp_cpu_kill_self(void)
/* Standard hot unplug procedure */

idle_task_exit();
- current->active_mm = NULL; /* for sanity */
cpu = smp_processor_id();
DBG("CPU%d offline\n", cpu);
generic_set_cpu_dead(cpu);
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index c49257a3b510..a132d875d351 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -49,6 +49,8 @@ static inline void mmdrop(struct mm_struct *mm)
__mmdrop(mm);
}

+void mmdrop(struct mm_struct *mm);
+
/*
* This has to be called after a get_task_mm()/mmget_not_zero()
* followed by taking the mmap_sem for writing before modifying the
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 2371292f30b0..244d30544377 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3,6 +3,7 @@
*
* This code is licenced under the GPL.
*/
+#include <linux/sched/mm.h>
#include <linux/proc_fs.h>
#include <linux/smp.h>
#include <linux/init.h>
@@ -564,6 +565,21 @@ static int bringup_cpu(unsigned int cpu)
return bringup_wait_for_ap(cpu);
}

+static int finish_cpu(unsigned int cpu)
+{
+ struct task_struct *idle = idle_thread_get(cpu);
+ struct mm_struct *mm = idle->active_mm;
+
+ /*
+ * idle_task_exit() will have switched to &init_mm, now
+ * clean up any remaining active_mm state.
+ */
+ if (mm != &init_mm)
+ idle->active_mm = &init_mm;
+ mmdrop(mm);
+ return 0;
+}
+
/*
* Hotplug state machine related functions
*/
@@ -1549,7 +1565,7 @@ static struct cpuhp_step cpuhp_hp_states[] = {
[CPUHP_BRINGUP_CPU] = {
.name = "cpu:bringup",
.startup.single = bringup_cpu,
- .teardown.single = NULL,
+ .teardown.single = finish_cpu,
.cant_stop = true,
},
/* Final state before CPU kills itself */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9a2fbf98fd6f..0bbf387d0f19 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6190,13 +6190,14 @@ void idle_task_exit(void)
struct mm_struct *mm = current->active_mm;

BUG_ON(cpu_online(smp_processor_id()));
+ BUG_ON(current != this_rq()->idle);

if (mm != &init_mm) {
switch_mm(mm, &init_mm, current);
- current->active_mm = &init_mm;
finish_arch_post_lock_switch();
}
- mmdrop(mm);
+
+ /* finish_cpu(), as ran on the BP, will clean up the active_mm state */
}

/*
--
2.25.1