Re: [PATCH v8 04/23] slab: add sheaf support for batching kfree_rcu() operations

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Sep 17, 2025 at 11:55:10AM +0200, Vlastimil Babka wrote:
> On 9/17/25 10:30, Harry Yoo wrote:
> > On Wed, Sep 10, 2025 at 10:01:06AM +0200, Vlastimil Babka wrote:
> >> +				sfw->skip = true;
> >> +				continue;
> >> +			}
> >>
> >> +			INIT_WORK(&sfw->work, flush_rcu_sheaf);
> >> +			sfw->skip = false;
> >> +			sfw->s = s;
> >> +			queue_work_on(cpu, flushwq, &sfw->work);
> >> +			flushed = true;
> >> +		}
> >> +
> >> +		for_each_online_cpu(cpu) {
> >> +			sfw = &per_cpu(slub_flush, cpu);
> >> +			if (sfw->skip)
> >> +				continue;
> >> +			flush_work(&sfw->work);
> >> +		}
> >> +
> >> +		mutex_unlock(&flush_lock);
> >> +	}
> >> +
> >> +	mutex_unlock(&slab_mutex);
> >> +	cpus_read_unlock();
> >> +
> >> +	if (flushed)
> >> +		rcu_barrier();
> > 
> > I think we need to call rcu_barrier() even if flushed == false?
> > 
> > Maybe a kvfree_rcu()'d object was already waiting for the rcu callback to
> > be processed before flush_all_rcu_sheaves() is called, and
> > in flush_all_rcu_sheaves() we skipped all (cache, cpu) pairs,
> > so flushed == false but the rcu callback isn't processed yet
> > by the end of the function?
> > 
> > That sounds like a very unlikely to happen in a realistic scenario,
> > but still possible...
> 
> Yes also good point, will flush unconditionally.
> 
> Maybe in __kfree_rcu_sheaf() I should also move the call_rcu(...) before
> local_unlock(). So we don't end up seeing a NULL pcs->rcu_free in
> flush_all_rcu_sheaves() because __kfree_rcu_sheaf() already set it to NULL,
> but didn't yet do the call_rcu() as it got preempted after local_unlock().

Makes sense to me.

> But then rcu_barrier() itself probably won't mean we make sure such cpus
> finished the local_locked section, if we didn't queue work on them. So maybe
> we need synchronize_rcu()?

Ah, it works because preemption disabled section works as a RCU
read-side critical section?

But then are we allowed to do release the local_lock to allocate empty
sheaves in __kfree_rcu_sheaf()?

-- 
Cheers,
Harry / Hyeonggon