[PATCH 0/6] swait: Introduce and use swq_has_sleepers

[Davidlohr Bueso <dave@stgolabs.net>]

Hi.

Recently[1] Nick mentioned that a lot of swait_active() callers look fishy.
This is because it inherited bad habits from regular waitqueues. Other than rcu,
kvm is one of the main callers, which I audited.

The following patches fix and/or justify (in baby steps) some of the
callers. Note that s390 and mips are other offenders, but I have no
idea if it can actually occur -- ie:

CPU0					CPU1
kvm_vcpu_block				kvm_mips_comparecount_func

  [S] prepare_to_swait(&vcpu->wq)					
					  [L] swait_active(&vcpu->wq)
					  [S] queue_timer_int(vcpu)

  [L] if (!kvm_vcpu_has_pending_timer(vcpu))
         schedule()


[1] swait: add missing barrier to swake_up:  https://lkml.org/lkml/2017/9/1/165

Thanks!

Davidlohr Bueso (6):
  sched/wait: Add swq_has_sleepers()
  kvm,async_pf: Use swq_has_sleepers()
  kvm,lapic: Justify use of swait_activate()
  x86,kvm: Fix apf_task_wake_one() serialization
  kvm: Serialize wq active checks in kvm_vcpu_wake_up()
  kvm,powerpc: Serialize wq active checks in ops->vcpu_kick

 arch/powerpc/kvm/book3s_hv.c |  2 +-
 arch/x86/kernel/kvm.c        |  2 +-
 arch/x86/kvm/lapic.c         |  6 +++++
 include/linux/swait.h        | 57 ++++++++++++++++++++++++++++++++++++++++++--
 virt/kvm/async_pf.c          |  6 +----
 virt/kvm/kvm_main.c          |  2 +-
 6 files changed, 65 insertions(+), 10 deletions(-)

-- 
2.12.0