[RFC PATCH 0/3] srcu: KVM: Add, export and use call_srcu_expedited()

[Sean Christopherson <seanjc@google.com>]

We've got a conundrum in KVM where we have multiple use cases that generally
want the same thing (eliminate waiting on guest configuration changes whenever
possible), but use KVM uAPIs in slightly different ways and effectively create
competing requirements.

The crux of the problem is that one use case wants KVM to free an object via
call_srcu() so that the task doesn't risk getting stalled waiting for a grace
period.  But for the other use case, using call_srcu() can trigger a
non-expedited grace period and cause a synchronize_srcu_expedited() in a
different ioctl (that must do a full sync, i.e. can't use call_srcu()) to stall
waiting for the non-expedited grace period.

Tagged RFC because while having the call_srcu() request do an expedited grace
period eliminates the unwanted synchronize_srcu_expedited() stalls, this feels
like a very crude fix.   That said, I'm definitely not opposed to this being a
final solution if it's the best option available.

Sean Christopherson (3):
  srcu: Declare exported symbols before including srcu{tiny,tree}.h
  srcu: Add and export call_srcu_expedited() to avoid transferring grace
    periods
  KVM: Expedite SRCU callbacks when freeing objects during I/O bus
    registration

 include/linux/srcu.h     | 10 +++++-----
 include/linux/srcutiny.h |  8 ++++++--
 include/linux/srcutree.h |  2 ++
 kernel/rcu/srcutree.c    |  7 +++++++
 virt/kvm/kvm_main.c      |  2 +-
 5 files changed, 21 insertions(+), 8 deletions(-)


base-commit: 5128b972fb2801ad9aca54d990a75611ab5283a9
-- 
2.53.0.473.g4a7958ca14-goog