Re: [PATCH] mm/mmu_notifier: Remove non_block_start/end() from notifier invocation

[Jason Gunthorpe]

On Tue, Aug 11, 2026 at 06:22:12PM +0100, David Woodhouse wrote:
> On Tue, 2026-08-11 at 13:24 -0300, Jason Gunthorpe wrote:
> > To be clear you should not be using any synchronize_[s]rcu() primitive
> > inside the invalidation callbacks. These are well known to have
> > multi-second delays on loaded systems which are a completely
> > inappropriate performance characteristic for these mm callbacks.
> > 
> > This statement has nothing to do with deadlock.
> > 
> > RCU is always a trade off, you can make the read side run really fast
> > and the write side is ghastly slow. If you can't handle the slow write
> > you shouldn't use RCU techniques.
> 
> The multi-second horror stories are about the *global* RCU/SRCU
> domains, where the grace period has to wait out arbitrary readers all
> over the kernel.
> 
> This is not that. It is a dedicated srcu_struct, private to one VM,
> and its entire reader population is a handful of KVM fast paths that
> until now were under irqsave rwlocks.

Are you sure? I've never heard that srcu has those kinds of properties.

If its so fast you should just propose a non-sleeping version and
leave the notifiers out of it

Jason