[tip: locking/core] zram: Use local lock to protect per-CPU data

["tip-bot2 for Mike Galbraith" <tip-bot2@linutronix.de>]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     19f545b6e07f753c4dc639c2f0ab52345733b6a8
Gitweb:        https://git.kernel.org/tip/19f545b6e07f753c4dc639c2f0ab52345733b6a8
Author:        Mike Galbraith <umgwanakikbuti@gmail.com>
AuthorDate:    Wed, 27 May 2020 22:11:19 +02:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Thu, 28 May 2020 10:31:10 +02:00

zram: Use local lock to protect per-CPU data

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]

Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20200527201119.1692513-8-bigeasy@linutronix.de
---
 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 912e3e6..5ee8e3f 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 72c2ee4..40f6420 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;