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

[Kunwu Chan <kunwu.chan@linux.dev>]

On 3/10/26 03:30, Sean Christopherson wrote:
> 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

Hi,

Thanks for writing this up.

The scenario you describe looks plausible.

That said, the cover letter wording might be a bit stronger than
current SRCU behavior warrants. A later synchronize_srcu_expedited()
can attempt to expedite an in-flight grace period, but it cannot
avoid delay already incurred (for example, if the GP has already
gone to sleep).

More generally, before adding an exported call_srcu_expedited()
helper, should we consider improving existing in-flight promotion
or delay behavior, or otherwise making the "expedite current GP"
case more explicit without introducing a new callback-facing API?


Thanx, Kunwu

>   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