Re: [PATCH] mm/slub: fix a deadlock in shuffle_freelist()

[Qian Cai <cai@lca.pw>]

On Mon, 2019-09-16 at 11:03 +0200, Sebastian Andrzej Siewior wrote:
> On 2019-09-13 12:27:44 [-0400], Qian Cai wrote:
> …
> > Chain exists of:
> >   random_write_wait.lock --> &rq->lock --> batched_entropy_u32.lock
> > 
> >  Possible unsafe locking scenario:
> > 
> >        CPU0                    CPU1
> >        ----                    ----
> >   lock(batched_entropy_u32.lock);
> >                                lock(&rq->lock);
> >                                lock(batched_entropy_u32.lock);
> >   lock(random_write_wait.lock);
> 
> would this deadlock still occur if lockdep knew that
> batched_entropy_u32.lock on CPU0 could be acquired at the same time
> as CPU1 acquired its batched_entropy_u32.lock?

I suppose that might fix it too if it can teach the lockdep the trick, but it
would be better if there is a patch if you have something in mind that could be
tested to make sure.