linux-kernel - Re: [PATCH] bpf: Fix preempt_enable_no

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Date:   Tue, 23 Apr 2019 13:27:29 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>,
        Roman Gushchin <guro@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH] bpf: Fix preempt_enable_no_resched() abuse

On Tue, Apr 23, 2019 at 12:56 PM Peter Zijlstra <peterz@...radead.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.

IOW, various variations of "if (need_resched())" exiting a loop, and
then outside the loop there's a cond_resched() or similar.

                    Linus