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Date: Fri, 30 Sep 2016 12:35:49 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: kernel test robot <xiaolong.ye@intel.com>
Cc: Con Kolivas <kernel@kolivas.org>, Ingo Molnar <mingo@kernel.org>,
        Alfred Chen <cchalpha@gmail.com>, LKML <linux-kernel@vger.kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>, lkp@01.org
Subject: Re: [lkp] [sched/core]  4fa5cd5245: BUG: sleeping function called
 from invalid context at kernel/locking/mutex.c:97
Message-ID: <20160930103549.GH5016@twins.programming.kicks-ass.net>
References: <20160930015102.GB20189@yexl-desktop>
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On Fri, Sep 30, 2016 at 09:51:02AM +0800, kernel test robot wrote:
> 
> FYI, we noticed the following commit:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
> commit 4fa5cd5245b627db88c9ca08ae442373b02596b4 ("sched/core: Do not use smp_processor_id() with preempt enabled in smpboot_thread_fn()")
> 

So its perfectly ok to use smp_processor_id() in preemptible context,
assuming the task is pinned to one cpu.

smpboot threads are per definition per-cpu threads, those are bound to a
specific CPU and affinity cannot be changed by userspace.