[PATCH v3 7/7] zram: Use local lock to protect per-CPU data

[Sebastian Andrzej Siewior]

From: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>

The zcomp driver uses per-CPU compression. The per-CPU data pointer is
acquired with get_cpu_ptr() which implicitly disables preemption.
It allocates memory inside the preempt disabled region which conflicts
with the PREEMPT_RT semantics.

Replace the implicit preemption control with an explicit local lock.
This allows RT kernels to substitute it with a real per CPU lock, which
serializes the access but keeps the code section preemptible. On non RT
kernels this maps to preempt_disable() as before, i.e. no functional
change.

[bigeasy: Use local_lock(), description, drop reordering]

Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Nitin Gupta <ngupta@xxxxxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
Signed-off-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 drivers/block/zram/zcomp.c | 7 +++++--
 drivers/block/zram/zcomp.h | 3 +++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index 912e3e63d8a09..5ee8e3fae5516 100644
--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -110,12 +110,13 @@ ssize_t zcomp_available_show(const char *comp, char *buf)
 
 struct zcomp_strm *zcomp_stream_get(struct zcomp *comp)
 {
-	return get_cpu_ptr(comp->stream);
+	local_lock(&comp->stream->lock);
+	return this_cpu_ptr(comp->stream);
 }
 
 void zcomp_stream_put(struct zcomp *comp)
 {
-	put_cpu_ptr(comp->stream);
+	local_unlock(&comp->stream->lock);
 }
 
 int zcomp_compress(struct zcomp_strm *zstrm,
@@ -159,6 +160,8 @@ int zcomp_cpu_up_prepare(unsigned int cpu, struct hlist_node *node)
 	int ret;
 
 	zstrm = per_cpu_ptr(comp->stream, cpu);
+	local_lock_init(&zstrm->lock);
+
 	ret = zcomp_strm_init(zstrm, comp);
 	if (ret)
 		pr_err("Can't allocate a compression stream\n");
diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
index 72c2ee4d843ed..40f6420f4b2e9 100644
--- a/drivers/block/zram/zcomp.h
+++ b/drivers/block/zram/zcomp.h
@@ -5,8 +5,11 @@
 
 #ifndef _ZCOMP_H_
 #define _ZCOMP_H_
+#include <linux/local_lock.h>
 
 struct zcomp_strm {
+	/* The members ->buffer and ->tfm are protected by ->lock. */
+	local_lock_t lock;
 	/* compression/decompression buffer */
 	void *buffer;
 	struct crypto_comp *tfm;
-- 
2.27.0.rc0