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Date:   Wed, 24 Apr 2019 15:19:54 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Waiman Long <longman@redhat.com>, 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
Message-ID: <20190424131954.GZ14281@hirez.programming.kicks-ass.net>
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On Tue, Apr 23, 2019 at 10:35:12PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 23, 2019 at 01:27:29PM -0700, Linus Torvalds wrote:
> > On Tue, Apr 23, 2019 at 12:56 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > 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.
> > 
> > That language may be a bit strong, or m,aybe the "implies it" might at
> > least be extended on.
> > 
> > It doesn't need to be "schedule()" per se, it can be any of the things
> > that check if we _need_ to be scheduled.
> 
> I'll try and word-smith that tomorrow, brain is fried. But yes,
> something that ends up in schedule() 'soon'.

I've made that:

Unless there is a call into schedule() in the immediate
(deterministic) future, 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.