From: Waiman Long <longman@redhat.com>
To: Zhenzhong Duan <zhenzhong.duan@oracle.com>,
LKML <linux-kernel@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
SRINIVAS <srinivas.eeda@oracle.com>
Subject: Re: Question about qspinlock nest
Date: Fri, 11 Jan 2019 08:59:35 -0800 [thread overview]
Message-ID: <4b43a3d1-a8f6-3ea7-7c32-b230d9fd0782@redhat.com> (raw)
In-Reply-To: <2eca6f60-3e8b-a389-27cb-8adbd9676607@oracle.com>
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.
In hindsight, we should have added a "BUG_ON(idx >= MAX_NODES);" check
to avoid silent corruption because of that issue.
Thanks,
Longman
next prev parent reply other threads:[~2019-01-11 16:59 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
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 [this message]
2019-01-10 20:03 ` Peter Zijlstra
2019-01-14 9:25 Zhenzhong Duan
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