Re: [PATCH v4 35/35] mm, slub: convert kmem_cpu_slab protection to local_lock

[Vlastimil Babka]

On 8/5/21 5:20 PM, Vlastimil Babka wrote:
> Embed local_lock into struct kmem_cpu_slab and use the irq-safe versions of
> local_lock instead of plain local_irq_save/restore. On !PREEMPT_RT that's
> equivalent, with better lockdep visibility. On PREEMPT_RT that means better
> preemption.
> 
> However, the cost on PREEMPT_RT is the loss of lockless fast paths which only
> work with cpu freelist. Those are designed to detect and recover from being
> preempted by other conflicting operations (both fast or slow path), but the
> slow path operations assume they cannot be preempted by a fast path operation,
> which is guaranteed naturally with disabled irqs. With local locks on
> PREEMPT_RT, the fast paths now also need to take the local lock to avoid races.
> 
> In the allocation fastpath slab_alloc_node() we can just defer to the slowpath
> __slab_alloc() which also works with cpu freelist, but under the local lock.
> In the free fastpath do_slab_free() we have to add a new local lock protected
> version of freeing to the cpu freelist, as the existing slowpath only works
> with the page freelist.
> 
> Also update the comment about locking scheme in SLUB to reflect changes done
> by this series.
> 
> [ Mike Galbraith <efault@xxxxxx>: use local_lock() without irq in PREEMPT_RT
>   scope; debugging of RT crashes resulting in put_cpu_partial() locking changes ]
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---

Another fixup. Is it too many and should we replace it all with a v5?
----8<----