[lkml]   [2019]   [Jan]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: Question about qspinlock nest
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.


 \ /
  Last update: 2019-01-11 18:00    [W:0.106 / U:1.112 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site