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

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Message-ID: <20190916090336.2mugbds4rrwxh6uz@linutronix.de>
Date:   Mon, 16 Sep 2019 11:03:36 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Qian Cai <cai@....pw>, 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 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?

Sebastian