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SubjectRe: [PATCH RT 4/6] rt/locking: Reenable migration accross schedule
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Rock #1..

hotplug/rt: Nest module_mutex inside cpu_hotplug.lock

PID: 11107 TASK: ffff8803b12b9900 CPU: 4 COMMAND: "stress-cpu-hotp"
#0 [ffff88038b34f9b8] __schedule at ffffffff816b7132
#1 [ffff88038b34fa08] schedule at ffffffff816b796b
#2 [ffff88038b34fa28] rt_mutex_slowlock at ffffffff816b93ee
#3 [ffff88038b34fac8] rt_mutex_fastlock at ffffffff811b0e9d
#4 [ffff88038b34faf0] rt_mutex_lock at ffffffff816b95c8
#5 [ffff88038b34fb08] _mutex_lock at ffffffff816baf59
#6 [ffff88038b34fb28] kernfs_find_and_get_ns at ffffffff812cd573
#7 [ffff88038b34fb50] sysfs_remove_group at ffffffff812d100a
#8 [ffff88038b34fb78] thermal_throttle_cpu_callback at ffffffff81036ab9
#9 [ffff88038b34fb98] notifier_call_chain at ffffffff8109b8dd
#10 [ffff88038b34fbd8] __raw_notifier_call_chain at ffffffff8109b90e
#11 [ffff88038b34fbe8] __cpu_notify at ffffffff81072825
#12 [ffff88038b34fbf8] cpu_notify_nofail at ffffffff81072b15
#13 [ffff88038b34fc08] notify_dead at ffffffff81072d06
#14 [ffff88038b34fc38] cpuhp_invoke_callback at ffffffff81073718
#15 [ffff88038b34fc78] cpuhp_down_callbacks at ffffffff81073a70
#16 [ffff88038b34fcb8] _cpu_down at ffffffff816afc61
#17 [ffff88038b34fd38] do_cpu_down at ffffffff8107434c
#18 [ffff88038b34fd60] cpu_down at ffffffff81074380
#19 [ffff88038b34fd70] cpu_subsys_offline at ffffffff814cd844
#20 [ffff88038b34fd80] device_offline at ffffffff814c7cca
#21 [ffff88038b34fda8] online_store at ffffffff814c7dc0
#22 [ffff88038b34fdd0] dev_attr_store at ffffffff814c4fb8
#23 [ffff88038b34fde0] sysfs_kf_write at ffffffff812cfbd4
#24 [ffff88038b34fe08] kernfs_fop_write at ffffffff812cf162
#25 [ffff88038b34fe50] __vfs_write at ffffffff81241418
#26 [ffff88038b34fed0] vfs_write at ffffffff81242525
#27 [ffff88038b34ff10] sys_write at ffffffff812438e9
#28 [ffff88038b34ff50] entry_SYSCALL_64_fastpath at ffffffff816bb4fc
RIP: 00007f05f3d69cd0 RSP: 00007ffdfc934468 RFLAGS: 00000246
RAX: ffffffffffffffda RBX: 0000000001908770 RCX: 00007f05f3d69cd0
RDX: 0000000000000002 RSI: 00007f05f4898000 RDI: 0000000000000001
RBP: 00007ffdfc934480 R8: 000000000000000a R9: 00007f05f4892700
R10: 00000000ffffffff R11: 0000000000000246 R12: 0000000000000007
R13: 0000000000000001 R14: 0000000000000009 R15: 000000000000000a
ORIG_RAX: 0000000000000001 CS: 0033 SS: 002b

stress-cpu-hotp blocks on kernfs_mutex, held by systemd-udevd..

crash> bt ffff8803b12bcb00
PID: 11130 TASK: ffff8803b12bcb00 CPU: 6 COMMAND: "systemd-udevd"
#0 [ffff88038b327a18] __schedule at ffffffff816b7132
#1 [ffff88038b327a68] schedule at ffffffff816b796b
#2 [ffff88038b327a88] rt_spin_lock_slowlock at ffffffff816b9750
#3 [ffff88038b327b30] rt_spin_lock_fastlock at ffffffff811b0f2c
#4 [ffff88038b327b50] rt_spin_lock__no_mg at ffffffff816bac7b
#5 [ffff88038b327b70] pin_current_cpu at ffffffff8107406a
#6 [ffff88038b327bb8] migrate_disable at ffffffff810a0e8e
#7 [ffff88038b327bd8] rt_spin_lock at ffffffff816badc9
#8 [ffff88038b327bf8] ida_simple_remove at ffffffff8138765c
#9 [ffff88038b327c18] kernfs_put at ffffffff812ccc58
#10 [ffff88038b327c60] __kernfs_remove at ffffffff812cd15c
#11 [ffff88038b327cc0] kernfs_remove_by_name_ns at ffffffff812ce2f3
#12 [ffff88038b327ce8] sysfs_remove_link at ffffffff812d05e9
#13 [ffff88038b327cf8] free_module at ffffffff8111c8f2
#14 [ffff88038b327d30] do_init_module at ffffffff811b157f
#15 [ffff88038b327d58] load_module at ffffffff8111f11b
#16 [ffff88038b327e98] SYSC_finit_module at ffffffff8111faf9
#17 [ffff88038b327f40] sys_finit_module at ffffffff8111fb3e
#18 [ffff88038b327f50] entry_SYSCALL_64_fastpath at ffffffff816bb4fc
RIP: 00007f75d9925f79 RSP: 00007ffd1c040ed8 RFLAGS: 00000246
RAX: ffffffffffffffda RBX: 0000000001d368e0 RCX: 00007f75d9925f79
RDX: 0000000000000000 RSI: 00007f75da0233c1 RDI: 0000000000000008
RBP: 0000000000000008 R8: 0000000000000000 R9: 0000000001d39c82
R10: 0000000000000008 R11: 0000000000000246 R12: 00007ffd1c03ff00
R13: 00007ffd1c03fee0 R14: 0000000000000005 R15: 000000000aba9500
ORIG_RAX: 0000000000000139 CS: 0033 SS: 002b

..which stress-cpu-hotp has blocked via pin_current_cpu(). Game Over.

Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
---
kernel/cpu.c | 9 +++++++++
1 file changed, 9 insertions(+)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -23,6 +23,9 @@
#include <linux/tick.h>
#include <linux/irq.h>
#include <trace/events/power.h>
+#ifdef CONFIG_PREEMPT_RT_BASE
+#include <linux/module.h>
+#endif

#include "smpboot.h"

@@ -442,10 +445,16 @@ void cpu_hotplug_begin(void)
schedule();
}
finish_wait(&cpu_hotplug.wq, &wait);
+#ifdef CONFIG_PREEMPT_RT_BASE
+ mutex_lock(&module_mutex);
+#endif
}

void cpu_hotplug_done(void)
{
+#ifdef CONFIG_PREEMPT_RT_BASE
+ mutex_unlock(&module_mutex);
+#endif
cpu_hotplug.active_writer = NULL;
mutex_unlock(&cpu_hotplug.lock);
cpuhp_lock_release();
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