Re: [patch V2 00/24] cpu/hotplug: Convert get_online_cpus() to a percpu_rwsem

Mark Rutland
2017-04-25T16:11:21Z

Hi,

This series appears to break boot on some arm64 platforms, seen with
next-20170424. More info below.

On Tue, Apr 18, 2017 at 07:04:42PM +0200, Thomas Gleixner wrote:
> get_online_cpus() is used in hot pathes in mainline and even more so in
> RT. That can show up badly under certain conditions because every locker
> contends on a global mutex. RT has it's own homebrewn mitigation which is
> an (badly done) open coded implementation of percpu_rwsems with recursion
> support.
>
> The proper replacement for that are percpu_rwsems, but that requires to
> remove recursion support.
>
> The conversion unearthed real locking issues which were previously not
> visible because the get_online_cpus() lockdep annotation was implemented
> with recursion support which prevents lockdep from tracking full dependency
> chains. These potential deadlocks are not related to recursive calls, they
> trigger on the first invocation because lockdep now has the full dependency
> chains available.

Catalin spotted next-20170424 wouldn't boot on a Juno system, where we see the
following splat (repeated forever) when we try to bring up the first secondary
CPU:

[ 0.213406] smp: Bringing up secondary CPUs ...
[ 0.250326] CPU features: enabling workaround for ARM erratum 832075
[ 0.250334] BUG: scheduling while atomic: swapper/1/0/0x00000002
[ 0.250337] Modules linked in:
[ 0.250346] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.11.0-rc7-next-20170424 #2
[ 0.250349] Hardware name: ARM Juno development board (r1) (DT)
[ 0.250353] Call trace:
[ 0.250365] [<ffff000008088510>] dump_backtrace+0x0/0x238
[ 0.250371] [<ffff00000808880c>] show_stack+0x14/0x20
[ 0.250377] [<ffff00000839d854>] dump_stack+0x9c/0xc0
[ 0.250384] [<ffff0000080e3540>] __schedule_bug+0x50/0x70
[ 0.250391] [<ffff000008932ecc>] __schedule+0x52c/0x5a8
[ 0.250395] [<ffff000008932f80>] schedule+0x38/0xa0
[ 0.250400] [<ffff000008935e8c>] rwsem_down_read_failed+0xc4/0x108
[ 0.250407] [<ffff0000080fe8e0>] __percpu_down_read+0x100/0x118
[ 0.250414] [<ffff0000080c0b60>] get_online_cpus+0x70/0x78
[ 0.250420] [<ffff0000081749e8>] static_key_enable+0x28/0x48
[ 0.250425] [<ffff00000808de90>] update_cpu_capabilities+0x78/0xf8
[ 0.250430] [<ffff00000808d14c>] update_cpu_errata_workarounds+0x1c/0x28
[ 0.250435] [<ffff00000808e004>] check_local_cpu_capabilities+0xf4/0x128
[ 0.250440] [<ffff00000808e894>] secondary_start_kernel+0x8c/0x118
[ 0.250444] [<000000008093d1b4>] 0x8093d1b4

I can reproduce this with the current head of the linux-tip smp/hotplug
branch (commit 77c60400c82bd993), with arm64 defconfig on a Juno R1
system.

When we bring the secondary CPU online, we detect an erratum that wasn't
present on the boot CPU, and try to enable a static branch we use to
track the erratum. The call to static_branch_enable() blows up as above.

I see that we now have static_branch_disable_cpuslocked(), but we don't
have an equivalent for enable. I'm not sure what we should be doing
here.

Thanks,
Mark.

> The following patch series addresses this by
>
> - Cleaning up places which call get_online_cpus() nested
>
> - Replacing a few instances with cpu_hotplug_disable() to prevent circular
> locking dependencies.
>
> The series depends on
>
> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git sched/core
> plus
> Linus tree merged in to avoid conflicts
>
> It's available in git from
>
> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git WIP.hotplug
>
> Changes since V1:
>
> - Fixed fallout reported by kbuild bot
> - Repaired the recursive call in perf
> - Repaired the interaction with jumplabels (Peter Zijlstra)
> - Renamed _locked to _cpuslocked
> - Picked up Acked-bys
>
> Thanks,
>
> tglx
>
> -------
> arch/arm/kernel/hw_breakpoint.c | 5
> arch/mips/kernel/jump_label.c | 2
> arch/powerpc/kvm/book3s_hv.c | 8 -
> arch/powerpc/platforms/powernv/subcore.c | 3
> arch/s390/kernel/time.c | 2
> arch/x86/events/core.c | 1
> arch/x86/events/intel/cqm.c | 12 -
> arch/x86/kernel/cpu/mtrr/main.c | 2
> b/arch/sparc/kernel/jump_label.c | 2
> b/arch/tile/kernel/jump_label.c | 2
> b/arch/x86/events/intel/core.c | 4
> b/arch/x86/kernel/jump_label.c | 2
> b/kernel/jump_label.c | 31 ++++-
> drivers/acpi/processor_driver.c | 4
> drivers/cpufreq/cpufreq.c | 9 -
> drivers/hwtracing/coresight/coresight-etm3x.c | 12 -
> drivers/hwtracing/coresight/coresight-etm4x.c | 12 -
> drivers/pci/pci-driver.c | 47 ++++---
> include/linux/cpu.h | 2
> include/linux/cpuhotplug.h | 29 ++++
> include/linux/jump_label.h | 3
> include/linux/padata.h | 3
> include/linux/pci.h | 1
> include/linux/stop_machine.h | 26 +++-
> kernel/cpu.c | 157 ++++++++------------------
> kernel/events/core.c | 9 -
> kernel/padata.c | 39 +++---
> kernel/stop_machine.c | 7 -
> 28 files changed, 228 insertions(+), 208 deletions(-)
>
>
>