linux-kernel - Question about qspinlock nest

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Message-ID: <e784a592-5dec-b0b3-6c70-d9ef775621d5@oracle.com>
Date:   Thu, 10 Jan 2019 16:02:53 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, longman@...hat.com,
        SRINIVAS <srinivas.eeda@...cle.com>
Subject: Question about qspinlock nest

Hi Maintainer,

There is a question confused me for days. Appreciate an answer.

In below code, the comment says we never have more than 4 nested contexts.

What happen if debug and mce exceptions nest with the four, or we ensure 
it never happen?

/*
  * Per-CPU queue node structures; we can never have more than 4 nested
  * contexts: task, softirq, hardirq, nmi.
  *
  * Exactly fits one 64-byte cacheline on a 64-bit architecture.
  *
  * PV doubles the storage and uses the second cacheline for PV state.
  */
static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[MAX_NODES]);

-- 
Thanks
Zhenzhong