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Date:   Fri, 19 Oct 2018 10:19:54 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Nadav Amit <namit@vmware.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        "Woodhouse, David" <dwmw@amazon.co.uk>
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix
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On Thu, Oct 18, 2018 at 09:29:39PM -0700, Andy Lutomirski wrote:

> > Another example is __BPF_PROG_RUN_ARRAY(), which also uses
> > preempt_enable_no_resched().
> 
> Alexei, I think this code is just wrong. Do you know why it uses
> preempt_enable_no_resched()?

Yes, that's a straight up bug.

It looks like I need to go fix up abuse again :/

> Oleg, the code in kernel/signal.c:
> 
>                 preempt_disable();
>                 read_unlock(&tasklist_lock);
>                 preempt_enable_no_resched();
>                 freezable_schedule();
> 

The purpose here is to avoid back-to-back schedule() calls, and this
pattern is one of the few correct uses of preempt_enable_no_resched().

Suppose we got a preemption while holding the read_lock(), then when
we'd do read_unlock(), we'd drop preempt_count to 0 and reschedule, then
when we get back we instantly call into schedule _again_.

What this code does, is it increments preempt_count such that
read_unlock() doesn't hit 0 and doesn't call schedule, then we lower it
to 0 without a call to schedule() and then call schedule() explicitly.