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From:   =?UTF-8?q?Tom=C3=A1=C5=A1=20Glozar?= <tglozar@gmail.com>
To:     nvdimm@lists.linux.dev
Cc:     dan.j.williams@intel.com, vishal.l.verma@intel.com,
        dave.jiang@intel.com, ira.weiny@intel.com,
        linux-kernel@vger.kernel.org, Tomas Glozar <tglozar@redhat.com>
Subject: [PATCH] nd_btt: Make BTT lanes preemptible
Date:   Tue, 12 Sep 2023 10:24:40 +0200
Message-ID: <20230912082440.325189-1-tglozar@gmail.com>
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From: Tomas Glozar <tglozar@redhat.com>

nd_region_acquire_lane uses get_cpu, which disables preemption. This is
an issue on PREEMPT_RT kernels, since btt_write_pg and also
nd_region_acquire_lane itself take a spin lock, resulting in BUG:
sleeping function called from invalid context.

Fix the issue by replacing get_cpu with smp_process_id and
migrate_disable when needed. This makes BTT operations preemptible, thus
permitting the use of spin_lock.

Fixes: 5212e11fde4d ("nd_btt: atomic sector updates")
Signed-off-by: Tomas Glozar <tglozar@redhat.com>
---
 drivers/nvdimm/region_devs.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
index 0a81f87f6f6c..e2f1fb99707f 100644
--- a/drivers/nvdimm/region_devs.c
+++ b/drivers/nvdimm/region_devs.c
@@ -939,7 +939,8 @@ unsigned int nd_region_acquire_lane(struct nd_region *nd_region)
 {
 	unsigned int cpu, lane;
 
-	cpu = get_cpu();
+	migrate_disable();
+	cpu = smp_processor_id();
 	if (nd_region->num_lanes < nr_cpu_ids) {
 		struct nd_percpu_lane *ndl_lock, *ndl_count;
 
@@ -958,16 +959,15 @@ 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();
+		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);
-		put_cpu();
 	}
-	put_cpu();
+	migrate_enable();
 }
 EXPORT_SYMBOL(nd_region_release_lane);
 
-- 
2.39.3