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Date:   Fri, 22 Nov 2019 21:01:22 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     linux-kernel@vger.kernel.org
Cc:     linux-tip-commits@vger.kernel.org,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Qian Cai <cai@lca.pw>, Thomas Gleixner <tglx@linutronix.de>,
        akpm@linux-foundation.org, cl@linux.com, keescook@chromium.org,
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        tytso@mit.edu, will@kernel.org, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [tip: sched/urgent] sched/core: Avoid spurious lock dependencies
Message-ID: <20191122200122.wx7ltij2w7w37cbe@linutronix.de>
References: <20191001091837.GK4536@hirez.programming.kicks-ass.net>
 <157363958888.29376.9190587096871610849.tip-bot2@tip-bot2>
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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