11:44 a.m.
-----Original Message-----
From: linux-kernel-owner(a)vger.kernel.org [mailto:linux-kernel-
owner(a)vger.kernel.org] On Behalf Of Sebastian Andrzej Siewior
Sent: Friday, March 8, 2019 17:42
To: Liu, Yongxin
Cc: linux-kernel(a)vger.kernel.org; linux-rt-users(a)vger.kernel.org;
tglx(a)linutronix.de; rostedt(a)goodmis.org; dan.j.williams(a)intel.com;
pagupta(a)redhat.com; Gortmaker, Paul; linux-nvdimm(a)lists.01.org
Subject: Re: [PATCH RT] nvdimm: make lane acquirement RT aware
On 2019-03-08 00:07:41 [+0000], Liu, Yongxin wrote:
> The lane is critical resource which needs to be protected. One CPU can
use only one
> lane. If CPU number is greater than the number of total lane, the lane
can be shared
> among CPUs.
>
> In non-RT kernel, get_cpu() disable preemption by calling
preempt_disable() first.
> Only one thread on the same CPU can get the lane.
>
> In RT kernel, if we only use raw_smp_processor_id(), this doesn't
protect the lane.
> Thus two threads on the same CPU can get the same lane at the same time.
>
> In this patch, two-level lock can avoid race condition for the lane.
but you still have the ndl_lock->lock which protects the resource. So in
the unlikely (but possible event) that you switch CPUs after obtaining
the CPU number you block on the lock. No harm is done, right?
The resource "lane" can be acquired recursively, so
"ndl_lock->lock" is a conditional lock.
ndl_count->count is per CPU.
ndl_lock->lock is per lane.
Here is an example:
Thread A on CPU 5 --> nd_region_acquire_lane --> lane# 5 --> get
"ndl_lock->lock"
--> nd_region_acquire_lane --> lane# 5 --> bypass "ndl_lock->lock"
due to "ndl_count->count++".
Thread B on CPU 5 --> nd_region_acquire_lane --> lane# 5 --> bypass
"ndl_lock->lock" ("ndl_count->count"
was changed by Thread A)
If we use raw_smp_processor_id(), no matter which CPU the thread was migrated to,
if there is another thread running on the old CPU, there will be race condition
due to per CPU variable "ndl_count->count".
Thanks,
Yongxin
Sebastian