Re: [patch V3 00/32] cpu/hotplug: Convert get_online_cpus() to a percpu_rwsem

[Ingo Molnar]

* Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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
> a (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.
> 
> 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 is on top of 4.12-rc2. It's available in git from
> 
>    git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git WIP.hotplug
> 
> Changes since V2:
> 
>   - Reworked the approach vs. perf/ftrace/kprobes, which simplified the lot
>   
>   - Renamed get_online_cpus() to cpus_read_lock() to reflect the nature of
>     the interface
> 
>   - Link the lockchains between hotplug control task and per cpu hotplug
>     threads and fixed the fallout of that.
> 
> Thanks,
> 
>         tglx
> 
> ---
>  arch/arm/kernel/hw_breakpoint.c               |   11 -
>  arch/arm/kernel/patch.c                       |    2 
>  arch/arm/probes/kprobes/core.c                |    3 
>  arch/arm64/include/asm/insn.h                 |    1 
>  arch/arm64/kernel/insn.c                      |    5 
>  arch/mips/kernel/jump_label.c                 |    2 
>  arch/powerpc/kvm/book3s_hv.c                  |   14 -
>  arch/powerpc/platforms/powernv/subcore.c      |    7 
>  arch/s390/kernel/jump_label.c                 |    2 
>  arch/s390/kernel/kprobes.c                    |    4 
>  arch/s390/kernel/time.c                       |    6 
>  arch/x86/events/core.c                        |    1 
>  arch/x86/events/intel/cqm.c                   |   16 -
>  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                |   11 -
>  b/arch/x86/kernel/jump_label.c                |    2 
>  b/kernel/jump_label.c                         |   20 +-
>  drivers/acpi/processor_driver.c               |    4 
>  drivers/acpi/processor_throttling.c           |   16 -
>  drivers/cpufreq/cpufreq.c                     |   21 +-
>  drivers/hwtracing/coresight/coresight-etm3x.c |   20 +-
>  drivers/hwtracing/coresight/coresight-etm4x.c |   20 +-
>  drivers/pci/pci-driver.c                      |   47 +++--
>  include/linux/cpu.h                           |   34 ++--
>  include/linux/cpuhotplug.h                    |   38 ++++
>  include/linux/padata.h                        |    3 
>  include/linux/pci.h                           |    1 
>  include/linux/perf_event.h                    |    2 
>  include/linux/sched.h                         |   10 +
>  include/linux/stop_machine.h                  |   26 ++-
>  kernel/cpu.c                                  |  213 +++++++++++---------------
>  kernel/events/core.c                          |  106 +++++++++---
>  kernel/kprobes.c                              |   59 +++----
>  kernel/padata.c                               |   43 ++---
>  kernel/stop_machine.c                         |   11 -
>  37 files changed, 444 insertions(+), 343 deletions(-)

Very nice work!

Modulo the open question about perf that Paul noticed:

Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>

Thanks,

	Ingo