linux-kernel - Re: [PATCH] mm/slub: fix a deadlock in shuffle

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Date:   Mon, 16 Sep 2019 10:01:27 -0400
From:   Qian Cai <cai@....pw>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        peterz@...radead.org, mingo@...hat.com
Cc:     akpm@...ux-foundation.org, tglx@...utronix.de, thgarnie@...gle.com,
        tytso@....edu, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, will@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, keescook@...omium.org
Subject: Re: [PATCH] mm/slub: fix a deadlock in shuffle_freelist()

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.