Commit 40fa3780 authored by Valentin Schneider's avatar Valentin Schneider Committed by Ingo Molnar

sched/core: Take the hotplug lock in sched_init_smp()

When running on linux-next (8c60c36d0b8c ("Add linux-next specific files
for 20181019")) + CONFIG_PROVE_LOCKING=y on a big.LITTLE system (e.g.
Juno or HiKey960), we get the following report:

 [    0.748225] Call trace:
 [    0.750685]  lockdep_assert_cpus_held+0x30/0x40
 [    0.755236]  static_key_enable_cpuslocked+0x20/0xc8
 [    0.760137]  build_sched_domains+0x1034/0x1108
 [    0.764601]  sched_init_domains+0x68/0x90
 [    0.768628]  sched_init_smp+0x30/0x80
 [    0.772309]  kernel_init_freeable+0x278/0x51c
 [    0.776685]  kernel_init+0x10/0x108
 [    0.780190]  ret_from_fork+0x10/0x18

The static_key in question is 'sched_asym_cpucapacity' introduced by

  df054e84 ("sched/topology: Add static_key for asymmetric CPU capacity optimizations")

In this particular case, we enable it because smp_prepare_cpus() will
end up fetching the capacity-dmips-mhz entry from the devicetree,
so we already have some asymmetry detected when entering sched_init_smp().

This didn't get detected in tip/sched/core because we were missing:

  commit cb538267 ("jump_label/lockdep: Assert we hold the hotplug lock for _cpuslocked() operations")

Calls to build_sched_domains() post sched_init_smp() will hold the
hotplug lock, it just so happens that this very first call is a
special case. As stated by a comment in sched_init_smp(), "There's no
userspace yet to cause hotplug operations" so this is a harmless

However, to both respect the semantics of underlying
callees and make lockdep happy, take the hotplug lock in
sched_init_smp(). This also satisfies the comment atop
sched_init_domains() that says "Callers must hold the hotplug lock".
Reported-by: 's avatarSudeep Holla <>
Tested-by: 's avatarSudeep Holla <>
Signed-off-by: 's avatarValentin Schneider <>
Signed-off-by: 's avatarPeter Zijlstra (Intel) <>
Cc: Linus Torvalds <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Link: 's avatarIngo Molnar <>
parent 993f0b05
......@@ -5859,11 +5859,14 @@ void __init sched_init_smp(void)
* There's no userspace yet to cause hotplug operations; hence all the
* CPU masks are stable and all blatant races in the below code cannot
* happen.
* happen. The hotplug lock is nevertheless taken to satisfy lockdep,
* but there won't be any contention on it.
/* Move init over to a non-isolated CPU */
if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment