linux-kernel - Re: Question about qspinlock nest

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Message-ID: <fa40ea62-5df8-4e18-af57-9ccd5c578e97@default>
Date:   Mon, 14 Jan 2019 01:25:41 -0800 (PST)
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     <longman@...hat.com>
Cc:     <srinivas.eeda@...cle.com>, <linux-kernel@...r.kernel.org>,
        <peterz@...radead.org>
Subject: Re: Question about qspinlock nest


----- longman@...hat.com wrote:

> On 01/11/2019 12:06 AM, Zhenzhong Duan wrote:
> >
> >
> > On 2019/1/10 22:43, Waiman Long wrote:
> >> On 01/10/2019 03:02 AM, Zhenzhong Duan wrote:
> >>> 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]);
> >>>
> >> Yes, both debug and mce exceptions are some kind of NMIs. So
> >> theoretically, it is possible to have more than four. Are you aware
> of
> >> any debug and MCE exception handlers that need to take a spinlock
> for
> >> synchronization?
> > Not for debug exception, for MCE exception handler I found below
> two:
> >
> > do_machine_check->mce_report_event->schedule_work
> > do_machine_check->force_sig->force_sig_info
> >
> > schedule_work() and force_sig_info() take spinlocks.
> > -- 
> > Thanks
> > Zhenzhong
> 
> The comment for do_machine_scheck() has already state that:
> 
>  * This is executed in NMI context not subject to normal locking
> rules. This
>  * implies that most kernel services cannot be safely used. Don't
> even
>  * think about putting a printk in there!
> 
> So even if it doesn't exceed the MAX_NODES limit, it could hit
> deadlock
> and other kind of locking hazards. So supporting MCE is not a reason
> strong enough to extend MAX_NODES.

Agree.

> 
> In hindsight, we should have added a "BUG_ON(idx >= MAX_NODES);"
> check
> to avoid silent corruption because of that issue.

Looks a good idea if it's hard to avoid using spinlock in MCE handler.

Thanks
Zhenzhong