[syzbot] possible deadlock in wake_up_all_idle_cpus

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 8006b911c90a Add linux-next specific files for 20211013
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=11e63434b00000
kernel config: https://syzkaller.appspot.com/x/.config?x=c7fd0a6a46d0131f
dashboard link: https://syzkaller.appspot.com/bug?extid=d5b23b18d2f4feae8a67
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d5b23b...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
5.15.0-rc5-next-20211013-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor.0/18539 is trying to acquire lock:
ffffffff8ba2e370 (cpu_hotplug_lock){++++}-{0:0}, at: wake_up_all_idle_cpus+0x13/0x80 kernel/smp.c:1173

but task is already holding lock:
ffff8880888aea28 (&mm->mmap_lock#2){++++}-{3:3}, at: mmap_write_lock_killable include/linux/mmap_lock.h:87 [inline]
ffff8880888aea28 (&mm->mmap_lock#2){++++}-{3:3}, at: vm_mmap_pgoff+0x15c/0x290 mm/util.c:517

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&mm->mmap_lock#2){++++}-{3:3}:
__might_fault mm/memory.c:5244 [inline]
__might_fault+0x104/0x170 mm/memory.c:5229
_copy_from_user+0x27/0x180 lib/usercopy.c:13
copy_from_user include/linux/uaccess.h:192 [inline]
perf_event_query_prog_array+0xfa/0x3d0 kernel/trace/bpf_trace.c:1890
_perf_ioctl+0x119/0x1f00 kernel/events/core.c:5694
perf_ioctl+0x76/0xb0 kernel/events/core.c:5730
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:874 [inline]
__se_sys_ioctl fs/ioctl.c:860 [inline]
__x64_sys_ioctl+0x193/0x200 fs/ioctl.c:860
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #2 (&cpuctx_mutex){+.+.}-{3:3}:
__mutex_lock_common kernel/locking/mutex.c:599 [inline]
__mutex_lock+0x12f/0x12f0 kernel/locking/mutex.c:732
perf_event_init_cpu+0x172/0x3e0 kernel/events/core.c:13295
perf_event_init+0x39d/0x408 kernel/events/core.c:13342
start_kernel+0x2bb/0x49b init/main.c:1057
secondary_startup_64_no_verify+0xb0/0xbb

-> #1 (pmus_lock){+.+.}-{3:3}:
__mutex_lock_common kernel/locking/mutex.c:599 [inline]
__mutex_lock+0x12f/0x12f0 kernel/locking/mutex.c:732
perf_event_init_cpu+0xc4/0x3e0 kernel/events/core.c:13289
cpuhp_invoke_callback+0x3b5/0x9a0 kernel/cpu.c:190
cpuhp_invoke_callback_range kernel/cpu.c:665 [inline]
cpuhp_up_callbacks kernel/cpu.c:693 [inline]
_cpu_up+0x3b0/0x790 kernel/cpu.c:1368
cpu_up kernel/cpu.c:1404 [inline]
cpu_up+0xfe/0x1a0 kernel/cpu.c:1376
bringup_nonboot_cpus+0xfe/0x130 kernel/cpu.c:1470
smp_init+0x2e/0x145 kernel/smp.c:1092
kernel_init_freeable+0x477/0x73a init/main.c:1612
kernel_init+0x1a/0x1d0 init/main.c:1509
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

-> #0 (cpu_hotplug_lock){++++}-{0:0}:
check_prev_add kernel/locking/lockdep.c:3063 [inline]
check_prevs_add kernel/locking/lockdep.c:3186 [inline]
validate_chain kernel/locking/lockdep.c:3801 [inline]
__lock_acquire+0x2a07/0x54a0 kernel/locking/lockdep.c:5027
lock_acquire kernel/locking/lockdep.c:5637 [inline]
lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5602
percpu_down_read include/linux/percpu-rwsem.h:51 [inline]
cpus_read_lock+0x3e/0x140 kernel/cpu.c:308
wake_up_all_idle_cpus+0x13/0x80 kernel/smp.c:1173
cpu_latency_qos_apply kernel/power/qos.c:249 [inline]
cpu_latency_qos_add_request+0x10d/0x370 kernel/power/qos.c:277
snd_pcm_hw_params+0x1399/0x1990 sound/core/pcm_native.c:787
snd_pcm_kernel_ioctl+0x164/0x310 sound/core/pcm_native.c:3363
snd_pcm_oss_change_params_locked+0x1936/0x3a60 sound/core/oss/pcm_oss.c:947
snd_pcm_oss_change_params sound/core/oss/pcm_oss.c:1091 [inline]
snd_pcm_oss_mmap+0x442/0x550 sound/core/oss/pcm_oss.c:2910
call_mmap include/linux/fs.h:2164 [inline]
mmap_region+0xd8c/0x1650 mm/mmap.c:1787
do_mmap+0x869/0xfb0 mm/mmap.c:1575
vm_mmap_pgoff+0x1b7/0x290 mm/util.c:519
ksys_mmap_pgoff+0x49f/0x620 mm/mmap.c:1624
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae

other info that might help us debug this:

Chain exists of:
cpu_hotplug_lock --> &cpuctx_mutex --> &mm->mmap_lock#2

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&mm->mmap_lock#2);
lock(&cpuctx_mutex);
lock(&mm->mmap_lock#2);
lock(cpu_hotplug_lock);

*** DEADLOCK ***

2 locks held by syz-executor.0/18539:
#0: ffff8880888aea28 (&mm->mmap_lock#2){++++}-{3:3}, at: mmap_write_lock_killable include/linux/mmap_lock.h:87 [inline]
#0: ffff8880888aea28 (&mm->mmap_lock#2){++++}-{3:3}, at: vm_mmap_pgoff+0x15c/0x290 mm/util.c:517
#1: ffff888018e76440 (&runtime->oss.params_lock){+.+.}-{3:3}, at: snd_pcm_oss_change_params sound/core/oss/pcm_oss.c:1086 [inline]
#1: ffff888018e76440 (&runtime->oss.params_lock){+.+.}-{3:3}, at: snd_pcm_oss_mmap+0x424/0x550 sound/core/oss/pcm_oss.c:2910

stack backtrace:
CPU: 0 PID: 18539 Comm: syz-executor.0 Not tainted 5.15.0-rc5-next-20211013-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
check_noncircular+0x25f/0x2e0 kernel/locking/lockdep.c:2143
check_prev_add kernel/locking/lockdep.c:3063 [inline]
check_prevs_add kernel/locking/lockdep.c:3186 [inline]
validate_chain kernel/locking/lockdep.c:3801 [inline]
__lock_acquire+0x2a07/0x54a0 kernel/locking/lockdep.c:5027
lock_acquire kernel/locking/lockdep.c:5637 [inline]
lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5602
percpu_down_read include/linux/percpu-rwsem.h:51 [inline]
cpus_read_lock+0x3e/0x140 kernel/cpu.c:308
wake_up_all_idle_cpus+0x13/0x80 kernel/smp.c:1173
cpu_latency_qos_apply kernel/power/qos.c:249 [inline]
cpu_latency_qos_add_request+0x10d/0x370 kernel/power/qos.c:277
snd_pcm_hw_params+0x1399/0x1990 sound/core/pcm_native.c:787
snd_pcm_kernel_ioctl+0x164/0x310 sound/core/pcm_native.c:3363
snd_pcm_oss_change_params_locked+0x1936/0x3a60 sound/core/oss/pcm_oss.c:947
snd_pcm_oss_change_params sound/core/oss/pcm_oss.c:1091 [inline]
snd_pcm_oss_mmap+0x442/0x550 sound/core/oss/pcm_oss.c:2910
call_mmap include/linux/fs.h:2164 [inline]
mmap_region+0xd8c/0x1650 mm/mmap.c:1787
do_mmap+0x869/0xfb0 mm/mmap.c:1575
vm_mmap_pgoff+0x1b7/0x290 mm/util.c:519
ksys_mmap_pgoff+0x49f/0x620 mm/mmap.c:1624
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7fcef50b8a39
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fcef262e188 EFLAGS: 00000246 ORIG_RAX: 0000000000000009
RAX: ffffffffffffffda RBX: 00007fcef51bbf60 RCX: 00007fcef50b8a39
RDX: 0000000001800003 RSI: 0000000000800000 RDI: 0000000020000000
RBP: 00007fcef5112c5f R08: 0000000000000004 R09: 0000000000000000
R10: 0000000000000012 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffda8d3cfcf R14: 00007fcef262e300 R15: 0000000000022000
</TASK>


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit: 7c832d2f9b95 Add linux-next specific files for 20211015
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=11aa8c78b00000
kernel config: https://syzkaller.appspot.com/x/.config?x=f6ac42766a768877

dashboard link: https://syzkaller.appspot.com/bug?extid=d5b23b18d2f4feae8a67
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14e920fcb00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13b83d80b00000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d5b23b...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected

5.15.0-rc5-next-20211015-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor097/6554 is trying to acquire lock:

ffffffff8ba2e370 (cpu_hotplug_lock){++++}-{0:0}, at: wake_up_all_idle_cpus+0x13/0x80 kernel/smp.c:1173

but task is already holding lock:

ffff888079003228 (&mm->mmap_lock#2){++++}-{3:3}, at: mmap_write_lock_killable include/linux/mmap_lock.h:87 [inline]
ffff888079003228 (&mm->mmap_lock#2){++++}-{3:3}, at: vm_mmap_pgoff+0x15c/0x290 mm/util.c:517

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&mm->mmap_lock#2){++++}-{3:3}:
__might_fault mm/memory.c:5244 [inline]
__might_fault+0x104/0x170 mm/memory.c:5229
_copy_from_user+0x27/0x180 lib/usercopy.c:13
copy_from_user include/linux/uaccess.h:192 [inline]

memdup_user+0x69/0xc0 mm/util.c:177
strndup_user+0x70/0xe0 mm/util.c:232
perf_event_set_filter kernel/events/core.c:10512 [inline]
_perf_ioctl+0x1a2/0x1f00 kernel/events/core.c:5659

perf_ioctl+0x76/0xb0 kernel/events/core.c:5730
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:874 [inline]
__se_sys_ioctl fs/ioctl.c:860 [inline]
__x64_sys_ioctl+0x193/0x200 fs/ioctl.c:860
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #2 (&cpuctx_mutex){+.+.}-{3:3}:
__mutex_lock_common kernel/locking/mutex.c:599 [inline]
__mutex_lock+0x12f/0x12f0 kernel/locking/mutex.c:732
perf_event_init_cpu+0x172/0x3e0 kernel/events/core.c:13295
perf_event_init+0x39d/0x408 kernel/events/core.c:13342

start_kernel+0x2bb/0x49b init/main.c:1059

secondary_startup_64_no_verify+0xb0/0xbb

-> #1 (pmus_lock){+.+.}-{3:3}:
__mutex_lock_common kernel/locking/mutex.c:599 [inline]
__mutex_lock+0x12f/0x12f0 kernel/locking/mutex.c:732
perf_event_init_cpu+0xc4/0x3e0 kernel/events/core.c:13289
cpuhp_invoke_callback+0x3b5/0x9a0 kernel/cpu.c:190
cpuhp_invoke_callback_range kernel/cpu.c:665 [inline]
cpuhp_up_callbacks kernel/cpu.c:693 [inline]
_cpu_up+0x3b0/0x790 kernel/cpu.c:1368
cpu_up kernel/cpu.c:1404 [inline]
cpu_up+0xfe/0x1a0 kernel/cpu.c:1376
bringup_nonboot_cpus+0xfe/0x130 kernel/cpu.c:1470
smp_init+0x2e/0x145 kernel/smp.c:1092

kernel_init_freeable+0x477/0x73a init/main.c:1614
kernel_init+0x1a/0x1d0 init/main.c:1511

ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

-> #0 (cpu_hotplug_lock){++++}-{0:0}:
check_prev_add kernel/locking/lockdep.c:3063 [inline]
check_prevs_add kernel/locking/lockdep.c:3186 [inline]
validate_chain kernel/locking/lockdep.c:3801 [inline]
__lock_acquire+0x2a07/0x54a0 kernel/locking/lockdep.c:5027
lock_acquire kernel/locking/lockdep.c:5637 [inline]
lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5602
percpu_down_read include/linux/percpu-rwsem.h:51 [inline]
cpus_read_lock+0x3e/0x140 kernel/cpu.c:308
wake_up_all_idle_cpus+0x13/0x80 kernel/smp.c:1173
cpu_latency_qos_apply kernel/power/qos.c:249 [inline]

cpu_latency_qos_remove_request.part.0+0xc4/0x2f0 kernel/power/qos.c:328
cpu_latency_qos_remove_request+0x65/0x80 kernel/power/qos.c:330
snd_pcm_hw_params+0x1481/0x1990 sound/core/pcm_native.c:784
snd_pcm_kernel_ioctl+0x164/0x310 sound/core/pcm_native.c:3355

snd_pcm_oss_change_params_locked+0x1936/0x3a60 sound/core/oss/pcm_oss.c:947
snd_pcm_oss_change_params sound/core/oss/pcm_oss.c:1091 [inline]
snd_pcm_oss_mmap+0x442/0x550 sound/core/oss/pcm_oss.c:2910
call_mmap include/linux/fs.h:2164 [inline]
mmap_region+0xd8c/0x1650 mm/mmap.c:1787
do_mmap+0x869/0xfb0 mm/mmap.c:1575
vm_mmap_pgoff+0x1b7/0x290 mm/util.c:519
ksys_mmap_pgoff+0x49f/0x620 mm/mmap.c:1624
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae

other info that might help us debug this:

Chain exists of:
cpu_hotplug_lock --> &cpuctx_mutex --> &mm->mmap_lock#2

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&mm->mmap_lock#2);
lock(&cpuctx_mutex);
lock(&mm->mmap_lock#2);
lock(cpu_hotplug_lock);

*** DEADLOCK ***

2 locks held by syz-executor097/6554:
#0: ffff888079003228 (&mm->mmap_lock#2){++++}-{3:3}, at: mmap_write_lock_killable include/linux/mmap_lock.h:87 [inline]
#0: ffff888079003228 (&mm->mmap_lock#2){++++}-{3:3}, at: vm_mmap_pgoff+0x15c/0x290 mm/util.c:517
#1: ffff88802317a440 (&runtime->oss.params_lock){+.+.}-{3:3}, at: snd_pcm_oss_change_params sound/core/oss/pcm_oss.c:1086 [inline]
#1: ffff88802317a440 (&runtime->oss.params_lock){+.+.}-{3:3}, at: snd_pcm_oss_mmap+0x424/0x550 sound/core/oss/pcm_oss.c:2910

stack backtrace:
CPU: 0 PID: 6554 Comm: syz-executor097 Not tainted 5.15.0-rc5-next-20211015-syzkaller #0

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
check_noncircular+0x25f/0x2e0 kernel/locking/lockdep.c:2143
check_prev_add kernel/locking/lockdep.c:3063 [inline]
check_prevs_add kernel/locking/lockdep.c:3186 [inline]
validate_chain kernel/locking/lockdep.c:3801 [inline]
__lock_acquire+0x2a07/0x54a0 kernel/locking/lockdep.c:5027
lock_acquire kernel/locking/lockdep.c:5637 [inline]
lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5602
percpu_down_read include/linux/percpu-rwsem.h:51 [inline]
cpus_read_lock+0x3e/0x140 kernel/cpu.c:308
wake_up_all_idle_cpus+0x13/0x80 kernel/smp.c:1173
cpu_latency_qos_apply kernel/power/qos.c:249 [inline]

cpu_latency_qos_remove_request.part.0+0xc4/0x2f0 kernel/power/qos.c:328
cpu_latency_qos_remove_request+0x65/0x80 kernel/power/qos.c:330
snd_pcm_hw_params+0x1481/0x1990 sound/core/pcm_native.c:784
snd_pcm_kernel_ioctl+0x164/0x310 sound/core/pcm_native.c:3355

snd_pcm_oss_change_params_locked+0x1936/0x3a60 sound/core/oss/pcm_oss.c:947
snd_pcm_oss_change_params sound/core/oss/pcm_oss.c:1091 [inline]
snd_pcm_oss_mmap+0x442/0x550 sound/core/oss/pcm_oss.c:2910
call_mmap include/linux/fs.h:2164 [inline]
mmap_region+0xd8c/0x1650 mm/mmap.c:1787
do_mmap+0x869/0xfb0 mm/mmap.c:1575
vm_mmap_pgoff+0x1b7/0x290 mm/util.c:519
ksys_mmap_pgoff+0x49f/0x620 mm/mmap.c:1624
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae

RIP: 0033:0x7fc0cb6221c9
Code: 28 c3 e8 2a 14 00 00 66 2e 0f 1f 84 00 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fffcd48ab28 EFLAGS: 00000246 ORIG_RAX: 0000000000000009
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fc0cb6221c9

RDX: 0000000001800003 RSI: 0000000000800000 RDI: 0000000020000000

RBP: 00007fc0cb5e61b0 R08: 0000000000000004 R09: 0000000000000000

[syzbot]

syzbot has bisected this issue to:

commit 8850cb663b5cda04d33f9cfbc38889d73d3c8e24
Author: Peter Zijlstra <pet...@infradead.org>
Date: Tue Sep 21 20:16:02 2021 +0000

sched: Simplify wake_up_*idle*()

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=117802fb300000
start commit: 7c832d2f9b95 Add linux-next specific files for 20211015
git tree: linux-next
final oops: https://syzkaller.appspot.com/x/report.txt?x=137802fb300000
console output: https://syzkaller.appspot.com/x/log.txt?x=157802fb300000

kernel config: https://syzkaller.appspot.com/x/.config?x=f6ac42766a768877
dashboard link: https://syzkaller.appspot.com/bug?extid=d5b23b18d2f4feae8a67

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14e920fcb00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13b83d80b00000

Reported-by: syzbot+d5b23b...@syzkaller.appspotmail.com
Fixes: 8850cb663b5c ("sched: Simplify wake_up_*idle*()")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

[Sebastian Andrzej Siewior]

Does this do anything useful?

Signed-off-by: Sebastian Andrzej Siewior <big...@linutronix.de>
---
kernel/smp.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index ad0b68a3a3d36..b3bf528fa14d3 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -1170,14 +1170,14 @@ void wake_up_all_idle_cpus(void)
{
int cpu;

- cpus_read_lock();
+ migrate_disable();
for_each_online_cpu(cpu) {
if (cpu == raw_smp_processor_id())
continue;

wake_up_if_idle(cpu);
}
- cpus_read_unlock();
+ migrate_enable();
}
EXPORT_SYMBOL_GPL(wake_up_all_idle_cpus);

--
2.33.0

[Peter Zijlstra]

On Tue, Oct 19, 2021 at 09:08:11PM +0200, Sebastian Andrzej Siewior wrote:
> Does this do anything useful?

I've got https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=sched/core&id=3dfddc2f0f76525218dd2d0c8725d9f21dd411e2

There's like 3 or 4 threads on this :/ I just got a tested-by on that.

[Sebastian Andrzej Siewior]

Yeah I saw a few emails and wanted to talk to the bot, assuming it would
keep quiet then.

for_each_possible_cpu looks big but is probably equal to online_cpu in
most cases.

Thanks ;)

Sebastian