linux-kernel - Re: mm: deadlock between get_online_cpus/pcpu

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Message-ID: <20170208141126.GY6515@twins.programming.kicks-ass.net>
Date:   Wed, 8 Feb 2017 15:11:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Michal Hocko <mhocko@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>, Tejun Heo <tj@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc

On Wed, Feb 08, 2017 at 02:03:32PM +0000, Mel Gorman wrote:
> > Yeah, we'll sort that out once it hits Linus tree and we move RT forward.
> > Though I have once complaint right away:
> > 
> > +	preempt_enable_no_resched();
> > 
> > This is a nono, even in mainline. You effectively disable a preemption
> > point.
> > 
> 
> This came up during review on whether it should or shouldn't be a preemption
> point. Initially it was preempt_enable() but a preemption point didn't
> exist before, the reviewer pushed for it and as it was the allocator fast
> path that was unlikely to need a reschedule or preempt, I made the change.

Not relevant. The only acceptable use of preempt_enable_no_resched() is
if the next statement is a schedule() variant.