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

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Date:   Tue, 23 Apr 2019 22:35:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.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 01:27:29PM -0700, Linus Torvalds wrote:
> 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.

I'll try and word-smith that tomorrow, brain is fried. But yes,
something that ends up in schedule() 'soon'.

The usage in ist_exit() is particularly 'fun'. That really should've had
a comment. That relies on the return-from-interrupt path.

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

That one 'funnily' doesn't actually work; cond_resched() is a no-op on
PREEMPT=y.