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SubjectRe: [PATCH RT] nvdimm: make lane acquirement RT aware
On 2019-03-11 00:44:58 [+0000], Liu, Yongxin wrote:
> > 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".

so I've been looking at it again. The recursive locking could have been
solved better. Like the local_lock() on -RT is doing it.
Given that you lock with preempt_disable() there should be no in-IRQ
usage.
But in the "nd_region->num_lanes >= nr_cpu_ids" case you don't take any
locks. That would be a problem with raw_smp_processor_id() approach.

So what about the completely untested patch here:

diff --git a/drivers/nvdimm/nd.h b/drivers/nvdimm/nd.h
index 379bf4305e615..98c2e9df4b2e4 100644
--- a/drivers/nvdimm/nd.h
+++ b/drivers/nvdimm/nd.h
@@ -109,7 +109,8 @@ unsigned sizeof_namespace_label(struct nvdimm_drvdata *ndd);
res; res = next, next = next ? next->sibling : NULL)

struct nd_percpu_lane {
- int count;
+ struct task_struct *owner;
+ int nestcnt;
spinlock_t lock;
};

diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
index e2818f94f2928..8a62f9833513f 100644
--- a/drivers/nvdimm/region_devs.c
+++ b/drivers/nvdimm/region_devs.c
@@ -946,19 +946,17 @@ int nd_blk_region_init(struct nd_region *nd_region)
*/
unsigned int nd_region_acquire_lane(struct nd_region *nd_region)
{
+ struct nd_percpu_lane *ndl_lock;
unsigned int cpu, lane;

- cpu = get_cpu();
- if (nd_region->num_lanes < nr_cpu_ids) {
- struct nd_percpu_lane *ndl_lock, *ndl_count;
-
- lane = cpu % nd_region->num_lanes;
- ndl_count = per_cpu_ptr(nd_region->lane, cpu);
- ndl_lock = per_cpu_ptr(nd_region->lane, lane);
- if (ndl_count->count++ == 0)
- spin_lock(&ndl_lock->lock);
- } else
- lane = cpu;
+ cpu = raw_smp_processor_id();
+ lane = cpu % nd_region->num_lanes;
+ ndl_lock = per_cpu_ptr(nd_region->lane, lane);
+ if (ndl_lock->owner != current) {
+ spin_lock(&ndl_lock->lock);
+ ndl_lock->owner = current;
+ }
+ ndl_lock->nestcnt++;

return lane;
}
@@ -966,17 +964,16 @@ EXPORT_SYMBOL(nd_region_acquire_lane);

void nd_region_release_lane(struct nd_region *nd_region, unsigned int lane)
{
- if (nd_region->num_lanes < nr_cpu_ids) {
- unsigned int cpu = get_cpu();
- struct nd_percpu_lane *ndl_lock, *ndl_count;
+ struct nd_percpu_lane *ndl_lock;

- ndl_count = per_cpu_ptr(nd_region->lane, cpu);
- ndl_lock = per_cpu_ptr(nd_region->lane, lane);
- if (--ndl_count->count == 0)
- spin_unlock(&ndl_lock->lock);
- put_cpu();
- }
- put_cpu();
+ ndl_lock = per_cpu_ptr(nd_region->lane, lane);
+ WARN_ON(ndl_lock->nestcnt == 0);
+ WARN_ON(ndl_lock->owner != current);
+ if (--ndl_lock->nestcnt)
+ return;
+
+ ndl_lock->owner = NULL;
+ spin_unlock(&ndl_lock->lock);
}
EXPORT_SYMBOL(nd_region_release_lane);

@@ -1042,7 +1039,8 @@ static struct nd_region *nd_region_create(struct nvdimm_bus *nvdimm_bus,

ndl = per_cpu_ptr(nd_region->lane, i);
spin_lock_init(&ndl->lock);
- ndl->count = 0;
+ ndl->owner = NULL;
+ ndl->nestcnt = 0;
}

for (i = 0; i < ndr_desc->num_mappings; i++) {
> Thanks,
> Yongxin

Sebastian

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