From: Zhenzhong Duan <zhenzhong.duan@oracle.com>
To: <longman@redhat.com>
Cc: <srinivas.eeda@oracle.com>, <linux-kernel@vger.kernel.org>,
<peterz@infradead.org>
Subject: Re: Question about qspinlock nest
Date: Mon, 14 Jan 2019 01:25:41 -0800 (PST) [thread overview]
Message-ID: <fa40ea62-5df8-4e18-af57-9ccd5c578e97@default> (raw)
----- longman@redhat.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
next reply other threads:[~2019-01-14 9:25 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-14 9:25 Zhenzhong Duan [this message]
-- strict thread matches above, loose matches on Subject: below --
2019-01-10 8:02 Question about qspinlock nest Zhenzhong Duan
2019-01-10 14:43 ` Waiman Long
2019-01-10 18:25 ` James Morse
2019-01-10 19:23 ` Waiman Long
2019-01-10 20:12 ` Peter Zijlstra
2019-01-11 18:32 ` James Morse
2019-01-14 13:16 ` Peter Zijlstra
2019-01-14 13:54 ` James Morse
2019-01-14 21:07 ` Waiman Long
2019-01-18 10:02 ` Peter Zijlstra
2019-01-18 10:24 ` Borislav Petkov
2019-01-18 14:50 ` Waiman Long
2019-01-18 20:06 ` Peter Zijlstra
2019-01-18 21:30 ` Waiman Long
[not found] ` <2eca6f60-3e8b-a389-27cb-8adbd9676607@oracle.com>
2019-01-11 9:16 ` Peter Zijlstra
2019-01-11 17:36 ` Borislav Petkov
2019-01-11 16:59 ` Waiman Long
2019-01-10 20:03 ` Peter Zijlstra
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