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Date:   Thu, 25 Apr 2019 14:23:40 -0700
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Waiman Long <longman@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>,
        Roman Gushchin <guro@fb.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [PATCH] bpf: Fix preempt_enable_no_resched() abuse
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On Tue, Apr 23, 2019 at 09:55:59PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 23, 2019 at 03:41:32PM -0400, Waiman Long wrote:
> > On 4/23/19 3:34 PM, Peter Zijlstra wrote:
> > > On Tue, Apr 23, 2019 at 03:12:16PM -0400, Waiman Long wrote:
> 
> > >> You are right on that. However, there is a variant called
> > >> preempt_enable_no_resched() that doesn't have this side effect. So I am
> > >> going to use that one instead.
> > > Only if the very next line is schedule(). Otherwise you're very much not
> > > going to use that function.
> > 
> > May I know the reason why. 
> 
> Because it can 'consume' a need_resched and introduces arbitrary delays
> before the schedule() eventually happens, breaking the very notion of
> PREEMPT=y (and the fundamentals RT relies on).
> 
> > I saw a number of instances of
> > preempt_enable_no_resched() without right next a schedule().
> 
> Look more closely.. and let me know, if true, those are bugs that need
> fixing.
> 
> Argghhh.. BPF...
> 
> Also, with the recent RCU rework, we can probably drop that
> rcu_read_lock()/rcu_read_unlock() from there if we're disabling
> preemption anyway.
> 
> ---
> Subject: bpf: Fix preempt_enable_no_resched() abuse
> 
> Unless the very next line is schedule(), or implies it, one must not use
> preempt_enable_no_resched(). It can cause a preemption to go missing and
> thereby cause arbitrary delays, breaking the PREEMPT=y invariant.
> 
> Cc: Roman Gushchin <guro@fb.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index f02367faa58d..944ccc310201 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -510,7 +510,7 @@ int bpf_prog_array_copy(struct bpf_prog_array __rcu *old_array,
>  		}					\
>  _out:							\
>  		rcu_read_unlock();			\
> -		preempt_enable_no_resched();		\
> +		preempt_enable();			\
>  		_ret;					\

Applied to bpf tree. Thanks!
It should have been fixed long ago. Not sure how we kept forgetting about it.