linux-kernel - Re: [tip: sched/urgent] sched/core: Avoid spurious lock dependencies

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Date:   Fri, 22 Nov 2019 21:01:22 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     linux-tip-commits@...r.kernel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Qian Cai <cai@....pw>, Thomas Gleixner <tglx@...utronix.de>,
        akpm@...ux-foundation.org, cl@...ux.com, keescook@...omium.org,
        penberg@...nel.org, rientjes@...gle.com, thgarnie@...gle.com,
        tytso@....edu, will@...nel.org, Ingo Molnar <mingo@...nel.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [tip: sched/urgent] sched/core: Avoid spurious lock dependencies

On 2019-11-13 10:06:28 [-0000], tip-bot2 for Peter Zijlstra wrote:
> sched/core: Avoid spurious lock dependencies
> 
> While seemingly harmless, __sched_fork() does hrtimer_init(), which,
> when DEBUG_OBJETS, can end up doing allocations.
> 
> This then results in the following lock order:
> 
>   rq->lock
>     zone->lock.rlock
>       batched_entropy_u64.lock
> 
> Which in turn causes deadlocks when we do wakeups while holding that
> batched_entropy lock -- as the random code does.

Peter, can it _really_ cause deadlocks? My understanding was that the
batched_entropy_u64.lock is a per-CPU lock and can _not_ cause a
deadlock because it can be always acquired on multiple CPUs
simultaneously (and it is never acquired cross-CPU).
Lockdep is simply not smart enough to see that and complains about it
like it would complain about a regular lock in this case.

Sebastian