zram: Replace bit spinlocks with spinlock_t for PREEMPT_RT.

Commit Message

Sebastian Andrzej Siewior June 9, 2022, 4:09 p.m. UTC

From: Mike Galbraith <[e-mail]>

The bit spinlock disables preemption on PREEMPT_RT. With disabled preemption it
is not allowed to acquire other sleeping locks which includes invoking
zs_free().

Use a spinlock_t on PREEMPT_RT for locking and set/ clear ZRAM_LOCK after the
lock has been acquired/ dropped.

Signed-off-by: Mike Galbraith <[e-mail]>
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
---

I'm simply forwarding Mike's patch here. The other alternative is to let
the driver depend on !PREEMPT_RT. I can't tell likely it is that this
driver is used. Mike most likely stumbled upon it while running LTP.

 drivers/block/zram/zram_drv.c |   36 ++++++++++++++++++++++++++++++++++++
 drivers/block/zram/zram_drv.h |    3 +++
 2 files changed, 39 insertions(+)
---

Patch

--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -58,6 +58,40 @@  static void zram_free_page(struct zram *
 static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
 				u32 index, int offset, struct bio *bio);
 
+#ifdef CONFIG_PREEMPT_RT
+static void zram_meta_init_table_locks(struct zram *zram, size_t num_pages)
+{
+	size_t index;
+
+	for (index = 0; index < num_pages; index++)
+		spin_lock_init(&zram->table[index].lock);
+}
+
+static int zram_slot_trylock(struct zram *zram, u32 index)
+{
+	int ret;
+
+	ret = spin_trylock(&zram->table[index].lock);
+	if (ret)
+		__set_bit(ZRAM_LOCK, &zram->table[index].flags);
+	return ret;
+}
+
+static void zram_slot_lock(struct zram *zram, u32 index)
+{
+	spin_lock(&zram->table[index].lock);
+	__set_bit(ZRAM_LOCK, &zram->table[index].flags);
+}
+
+static void zram_slot_unlock(struct zram *zram, u32 index)
+{
+	__clear_bit(ZRAM_LOCK, &zram->table[index].flags);
+	spin_unlock(&zram->table[index].lock);
+}
+
+#else
+
+static void zram_meta_init_table_locks(struct zram *zram, size_t num_pages) { }
 
 static int zram_slot_trylock(struct zram *zram, u32 index)
 {
@@ -73,6 +107,7 @@  static void zram_slot_unlock(struct zram
 {
 	bit_spin_unlock(ZRAM_LOCK, &zram->table[index].flags);
 }
+#endif
 
 static inline bool init_done(struct zram *zram)
 {
@@ -1195,6 +1230,7 @@  static bool zram_meta_alloc(struct zram
 
 	if (!huge_class_size)
 		huge_class_size = zs_huge_class_size(zram->mem_pool);
+	zram_meta_init_table_locks(zram, num_pages);
 	return true;
 }
 
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -63,6 +63,9 @@  struct zram_table_entry {
 		unsigned long element;
 	};
 	unsigned long flags;
+#ifdef CONFIG_PREEMPT_RT
+	spinlock_t lock;
+#endif
 #ifdef CONFIG_ZRAM_MEMORY_TRACKING
 	ktime_t ac_time;
 #endif