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(-)