[PATCH v3] nvdimm/btt: Handle preemption in BTT lane acquisition

[PATCH v3] nvdimm/btt: Handle preemption in BTT lane acquisition

[Alison Schofield]

BTT lanes serialize access to per-lane metadata and workspace state
during BTT I/O. The btt-check unit test reports data mismatches during
BTT writes due to a race in lane acquisition that can lead to silent
data corruption.

The existing lane model uses a spinlock together with a per-CPU
recursion count. That recursion model stopped being valid after BTT
lanes became preemptible: another task can run on the same CPU,
observe a non-zero recursion count, bypass locking, and use the same
lane concurrently.

BTT lanes are also held across metadata and data updates that can
reach nvdimm_flush(). Some provider flush callbacks can sleep, making
a spinlock the wrong primitive for the lane lifetime. That issue
predates this fix, but becomes more visible now that BTT lanes are
preemptible.

Replace the spinlock with a per-lane mutex, remove the per-CPU
recursion fast path, and take the lane lock unconditionally.

Add might_sleep() to catch any future atomic-context caller.

Found with the ndctl unit test btt-check.sh.

Fixes: 36c75ce3bd29 ("nd_btt: Make BTT lanes preemptible")
Assisted-by: Claude Sonnet 4.5
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
---

Aboorva: I appreciate your Tested-by tag, yet due to the churn,
I did not apply it. Please re-test with this version.

Changes in v3:
- Replace spinlock with a per-lane mutex (Arboorva)*
- Rebase onto 7.1-rc1
- Update commit log

*Arboorva pointed out that BTT write-side lane ownership can reach
provider flush callbacks that may sleep, making the existing
spinlock-based lane lifetime invalid. My initial thought was to
create a small series where the first patch converts the per-lane
lock to a mutex so the lane critical section can safely sleep.
That left an intermediate bad state, so the changes are kept
together in this single patch.

Changes in v2:
Use spin_(un)lock_bh() (Sashiko AI)
Update commit log per softirq re-enty and spinlock change

A new unit test to stress this is under review here:
https://lore.kernel.org/nvdimm/20260424233633.3762217-1-alison.schofield@intel.com/


 drivers/nvdimm/nd.h          |  3 +--
 drivers/nvdimm/region_devs.c | 50 ++++++++++--------------------------
 2 files changed, 14 insertions(+), 39 deletions(-)

diff --git a/drivers/nvdimm/nd.h b/drivers/nvdimm/nd.h
index b199eea3260e..3fbeaddb5b5c 100644
--- a/drivers/nvdimm/nd.h
+++ b/drivers/nvdimm/nd.h
@@ -366,8 +366,7 @@ unsigned sizeof_namespace_label(struct nvdimm_drvdata *ndd);
 			res; res = next, next = next ? next->sibling : NULL)
 
 struct nd_percpu_lane {
-	int count;
-	spinlock_t lock;
+	struct mutex lock; /* serialize lane access */
 };
 
 enum nd_label_flags {
diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
index e35c2e18518f..d01b16f6a463 100644
--- a/drivers/nvdimm/region_devs.c
+++ b/drivers/nvdimm/region_devs.c
@@ -904,52 +904,33 @@ void nd_region_advance_seeds(struct nd_region *nd_region, struct device *dev)
  * nd_region_acquire_lane - allocate and lock a lane
  * @nd_region: region id and number of lanes possible
  *
- * A lane correlates to a BLK-data-window and/or a log slot in the BTT.
- * We optimize for the common case where there are 256 lanes, one
- * per-cpu.  For larger systems we need to lock to share lanes.  For now
- * this implementation assumes the cost of maintaining an allocator for
- * free lanes is on the order of the lock hold time, so it implements a
- * static lane = cpu % num_lanes mapping.
+ * A lane correlates to a log slot in the BTT. Lanes are shared across
+ * CPUs using a static lane = cpu % num_lanes mapping, with a per-lane
+ * mutex to serialize access.
  *
- * In the case of a BTT instance on top of a BLK namespace a lane may be
- * acquired recursively.  We lock on the first instance.
- *
- * In the case of a BTT instance on top of PMEM, we only acquire a lane
- * for the BTT metadata updates.
+ * Callers must be in sleepable context. The only in-tree caller is
+ * BTT's ->submit_bio handler (btt_read_pg / btt_write_pg).
  */
 unsigned int nd_region_acquire_lane(struct nd_region *nd_region)
 {
 	unsigned int cpu, lane;
 
-	migrate_disable();
-	cpu = smp_processor_id();
-	if (nd_region->num_lanes < nr_cpu_ids) {
-		struct nd_percpu_lane *ndl_lock, *ndl_count;
+	might_sleep();
 
+	cpu = raw_smp_processor_id();
+	if (nd_region->num_lanes < nr_cpu_ids)
 		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
+	else
 		lane = cpu;
 
+	mutex_lock(&per_cpu_ptr(nd_region->lane, lane)->lock);
 	return lane;
 }
 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 = smp_processor_id();
-		struct nd_percpu_lane *ndl_lock, *ndl_count;
-
-		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);
-	}
-	migrate_enable();
+	mutex_unlock(&per_cpu_ptr(nd_region->lane, lane)->lock);
 }
 EXPORT_SYMBOL(nd_region_release_lane);
 
@@ -1023,13 +1004,8 @@ static struct nd_region *nd_region_create(struct nvdimm_bus *nvdimm_bus,
 	if (!nd_region->lane)
 		goto err_percpu;
 
-        for (i = 0; i < nr_cpu_ids; i++) {
-		struct nd_percpu_lane *ndl;
-
-		ndl = per_cpu_ptr(nd_region->lane, i);
-		spin_lock_init(&ndl->lock);
-		ndl->count = 0;
-	}
+	for (i = 0; i < nr_cpu_ids; i++)
+		mutex_init(&per_cpu_ptr(nd_region->lane, i)->lock);
 
 	for (i = 0; i < ndr_desc->num_mappings; i++) {
 		struct nd_mapping_desc *mapping = &ndr_desc->mapping[i];

base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
-- 
2.37.3

Re: [PATCH v3] nvdimm/btt: Handle preemption in BTT lane acquisition

[Alison Schofield]

On Wed, May 13, 2026 at 05:23:12PM -0700, Alison Schofield wrote:
> BTT lanes serialize access to per-lane metadata and workspace state
> during BTT I/O. The btt-check unit test reports data mismatches during
> BTT writes due to a race in lane acquisition that can lead to silent
> data corruption.
> 
> The existing lane model uses a spinlock together with a per-CPU
> recursion count. That recursion model stopped being valid after BTT
> lanes became preemptible: another task can run on the same CPU,
> observe a non-zero recursion count, bypass locking, and use the same
> lane concurrently.
> 
> BTT lanes are also held across metadata and data updates that can
> reach nvdimm_flush(). Some provider flush callbacks can sleep, making
> a spinlock the wrong primitive for the lane lifetime. That issue
> predates this fix, but becomes more visible now that BTT lanes are
> preemptible.
> 
> Replace the spinlock with a per-lane mutex, remove the per-CPU
> recursion fast path, and take the lane lock unconditionally.
> 
> Add might_sleep() to catch any future atomic-context caller.
> 
> Found with the ndctl unit test btt-check.sh.
> 
> Fixes: 36c75ce3bd29 ("nd_btt: Make BTT lanes preemptible")
> Assisted-by: Claude Sonnet 4.5
> Signed-off-by: Alison Schofield <alison.schofield@intel.com>
> ---

Sashiko review offered applicable feedback. With the recursion count
removed, the lanes are really just a lock pool indexed by lane number,
so the per-cpu allocation no longer makes sense.

Working a v4 where pre-CPU lane storage gets replaced with a dynamically
allocated per-lane mutex array.

https://sashiko.dev/#/patchset/20260514002314.65024-1-alison.schofield%40intel.com

snip