linux-kernel - Re: Question about qspinlock nest

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Message-ID: <20190110201217.GH2861@worktop.programming.kicks-ass.net>
Date:   Thu, 10 Jan 2019 21:12:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     James Morse <james.morse@....com>
Cc:     Waiman Long <longman@...hat.com>,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        SRINIVAS <srinivas.eeda@...cle.com>
Subject: Re: Question about qspinlock nest

On Thu, Jan 10, 2019 at 06:25:57PM +0000, James Morse wrote:

> On arm64 if all the RAS and psuedo-NMI patches land, our worst-case interleaving
> jumps to at least 7. The culprit is APEI using spinlocks to protect fixmap slots.
> 
> I have an RFC to bump the number of node bits from 2 to 3, but as this is APEI
> four times, it may be preferable to make it use something other than spinlocks.
> 
> 
> The worst-case order is below. Each one masks those before it:
> 1. process context
> 2. soft-irq
> 3. hard-irq
> 4. psuedo-nmi [0]
>    - using the irqchip priorities to configure some IRQs as NMI.
> 5. SError [1]
>    - a bit like an asynchronous MCE. ACPI allows this to convey CPER records,
>      requiring an APEI call.
> 6&7. SDEI [2]
>      - a firmware triggered software interrupt, only its two of them, either of
>        which could convey CPER records.
> 8. Synchronous external abort
>    - again, similar to MCE. There are systems using this with APEI.

The thing is, everything non-maskable (NMI like) really should not be
using spinlocks at all.

I otherwise have no clue about wth APEI is, but it sounds like horrible
crap ;-)