Re: [PATCH v2] sched/core: fix illegal RCU from offline CPUs

From: Qian Cai
Date: Sun Mar 29 2020 - 22:42:35 EST

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> On Jan 21, 2020, at 5:35 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Jan 20, 2020 at 03:35:09PM -0500, Qian Cai wrote:
>>> On Jan 20, 2020, at 5:17 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>>
>>> Bah.. that's horrible. Surely we can find a better place to do this in
>>> the whole hotplug machinery.
>>>
>>> Perhaps you can have takedown_cpu() do the mmdrop()?
>>
>> The problem is that no all arch_cpu_idle_dead() will call
>> idle_task_exit(). For example, alpha and parisc are not, so it needs
>
> How is that not broken? If the idle thread runs on an active_mm, we need
> to drop that reference. This needs fixing regardless.

It turns out alpha does not support cpu hotplug (no CONFIG_HOTPLUG_CPU),
and parisc will use &init_mm for idle tasks of secondary CPUs as in,

smp_callin()
smp_cpu_init()

/* Initialise the idle task for this CPU */
mmgrab(&init_mm);
current->active_mm = &init_mm;
BUG_ON(current->mm);
enter_lazy_tlb(&init_mm, current);

>
>> to deal with some kind of ifdef dance in takedown_cpu() to
>> conditionally call mmdrop() which sounds even more horrible?
>>
>> If you really prefer it anyway, maybe something like touching every archâs __cpu_die() to also call mmdrop() depends on arches?
>>
>> BTW, how to obtain the other CPUâs current task mm? Is that cpu_rq(cpu)->curr->active_mm?
>
> Something like this; except you'll need to go audit archs to make sure
> they all call idle_task_exit() and/or put in comments on why they don't
> have to (perhaps their bringup switches them to &init_mm unconditionally
> and the switch_mm() is not required).

I have to modify your code slightly (below) to pass compilation, but then it will
panic during cpu offline for some reasons,

[ 258.972625][ T9006] Faulting instruction address: 0xc00000000010afc4
[ 258.972640][ T9006] Oops: Kernel access of bad area, sig: 11 [#1]
[ 258.972651][ T9006] LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=256 DEBUG_PAGEALLOC NUMA PowerNV
[ 258.972675][ T9006] Modules linked in: kvm_hv kvm ip_tables x_tables xfs sd_mod bnx2x ahci mdio libahci tg3 libata libphy firmware_class dm_mirror dm_region_hash dm_log dm_mod
[ 258.972716][ T9006] CPU: 84 PID: 9006 Comm: cpuhotplug04.sh Not tainted 5.6.0-rc7-next-20200327+ #7
[ 258.972741][ T9006] NIP: c00000000010afc4 LR: c00000000010afa0 CTR: fffffffffffffffd
[ 258.972775][ T9006] REGS: c000201a54def7f0 TRAP: 0300 Not tainted (5.6.0-rc7-next-20200327+)
[ 258.972809][ T9006] MSR: 900000000280b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE> CR: 28224824 XER: 20040000
[ 258.972839][ T9006] CFAR: c00000000015951c DAR: 0000000000000050 DSISR: 40000000 IRQMASK: 0
[ 258.972839][ T9006] GPR00: c00000000010afa0 c000201a54defa80 c00000000165bd00 c000000001604180
[ 258.972839][ T9006] GPR04: c00000008160496f 0000000000000000 ffff0a01ffffff10 0000000000000020
[ 258.972839][ T9006] GPR08: 0000000000000050 0000000000000000 c00000000162c0e0 0000000000000002
[ 258.972839][ T9006] GPR12: 0000000088224428 c000201fff69a900 0000000040000000 000000011a029798
[ 258.972839][ T9006] GPR16: 000000011a029724 0000000119fc6968 0000000119f5f230 000000011a02d568
[ 258.972839][ T9006] GPR20: 00000001554c66f0 0000000000000000 c000001ffc957820 c00000000010af80
[ 258.972839][ T9006] GPR24: 0000001ffb8a0000 c0000000014f34a8 0000000000000056 c0000000010b7820
[ 258.972839][ T9006] GPR28: c00000000168c654 0000000000000000 c00000000168c3a8 0000000000000000
[ 258.973070][ T9006] NIP [c00000000010afc4] finish_cpu+0x44/0x90
atomic_dec_return_relaxed at arch/powerpc/include/asm/atomic.h:179
(inlined by) atomic_dec_return at include/linux/atomic-fallback.h:518
(inlined by) atomic_dec_and_test at include/linux/atomic-fallback.h:1035
(inlined by) mmdrop at include/linux/sched/mm.h:48
(inlined by) finish_cpu at kernel/cpu.c:579
[ 258.973092][ T9006] LR [c00000000010afa0] finish_cpu+0x20/0x90
[ 258.973113][ T9006] Call Trace:
[ 258.973132][ T9006] [c000201a54defa80] [c00000000010afa0] finish_cpu+0x20/0x90 (unreliable)
[ 258.973166][ T9006] [c000201a54defaa0] [c00000000010cd34] cpuhp_invoke_callback+0x194/0x15f0
cpuhp_invoke_callback at kernel/cpu.c:173
[ 258.973202][ T9006] [c000201a54defb50] [c000000000111acc] _cpu_down.constprop.15+0x17c/0x410
[ 258.973237][ T9006] [c000201a54defbc0] [c00000000010ff84] cpu_down+0x64/0xb0
[ 258.973263][ T9006] [c000201a54defc00] [c000000000754760] cpu_subsys_offline+0x20/0x40
[ 258.973299][ T9006] [c000201a54defc20] [c00000000074ab10] device_offline+0x100/0x140
[ 258.973344][ T9006] [c000201a54defc60] [c00000000074ace0] online_store+0xa0/0x110
[ 258.973378][ T9006] [c000201a54defcb0] [c000000000744508] dev_attr_store+0x38/0x60
[ 258.973414][ T9006] [c000201a54defcd0] [c0000000005df770] sysfs_kf_write+0x70/0xb0
[ 258.973438][ T9006] [c000201a54defd10] [c0000000005de92c] kernfs_fop_write+0x11c/0x270
[ 258.973463][ T9006] [c000201a54defd60] [c0000000004c09bc] __vfs_write+0x3c/0x70
[ 258.973487][ T9006] [c000201a54defd80] [c0000000004c3dbc] vfs_write+0xcc/0x200
[ 258.973522][ T9006] [c000201a54defdd0] [c0000000004c415c] ksys_write+0x7c/0x140
[ 258.973557][ T9006] [c000201a54defe20] [c00000000000b3f8] system_call+0x5c/0x68
[ 258.973579][ T9006] Instruction dump:
[ 258.973598][ T9006] f8010010 f821ffe1 4804e4ed 60000000 3d42fffd 394a03e0 e9230560 7fa95000
[ 258.973636][ T9006] 419e0030 f9430560 39090050 7c0004ac <7d404028> 314affff 7d40412d 40c2fff4
[ 258.973675][ T9006] ---[ end trace 4f72c711066ac397 ]---
[ 259.076271][ T9006]
[ 260.076362][ T9006] Kernel panic - not syncing: Fatal exception

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index c49257a3b510..bc782562ae93 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);
}

+extern 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 0cca1a2b4930..302007a602a5 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 +565,22 @@ 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 +1566,7 @@ 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 ac8b65298774..16aad39216a2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6248,13 +6248,14 @@ 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 */
}

/*

>
> ---
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 9c706af713fb..2b4d8a69e8d9 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -564,6 +564,23 @@ 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)
> + return;
> +
> + idle->active_mm = &init_mm;
> + mmdrop(mm);
> +}
> +
> /*
> * Hotplug state machine related functions
> */
> @@ -1434,7 +1451,7 @@ 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 fc1dfc007604..8f049fb77a3d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6188,13 +6188,14 @@ 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 */
> }
>
> /*

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