linux-kernel - [PATCH] zram: Replace bit spinlocks with spinlock_t for PREEMPT

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Date: Wed, 19 Jun 2024 17:08:14 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Jens Axboe <axboe@...nel.dk>, Mike Galbraith <umgwanakikbuti@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] zram: Replace bit spinlocks with spinlock_t for PREEMPT_RT.

From: Mike Galbraith <umgwanakikbuti@...il.com>

The bit spinlock disables preemption. The spinlock_t lock becomes a sleeping
lock on PREEMPT_RT and it can not be acquired in this context. In this locked
section, zs_free() acquires a zs_pool::lock, and there is access to
zram::wb_limit_lock.

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

Signed-off-by: Mike Galbraith <umgwanakikbuti@...il.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
I posted this few times. Mikes intents to keep it based on last feedback.
Any reason not to apply it?

https://lkml.kernel.org/r/YqIbMuHCPiQk+Ac2@linutronix.de
https://lore.kernel.org/20230323161830.jFbWCosd@linutronix.de

 drivers/block/zram/zram_drv.c |   37 +++++++++++++++++++++++++++++++++++++
 drivers/block/zram/zram_drv.h |    3 +++
 2 files changed, 40 insertions(+)

--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -57,6 +57,41 @@ static void zram_free_page(struct zram *
 static int zram_read_page(struct zram *zram, struct page *page, u32 index,
 			  struct bio *parent);
 
+#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)
 {
 	return bit_spin_trylock(ZRAM_LOCK, &zram->table[index].flags);
@@ -71,6 +106,7 @@ 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)
 {
@@ -1226,6 +1262,7 @@ 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
@@ -69,6 +69,9 @@ struct zram_table_entry {
 		unsigned long element;
 	};
 	unsigned long flags;
+#ifdef CONFIG_PREEMPT_RT
+	spinlock_t lock;
+#endif
 #ifdef CONFIG_ZRAM_TRACK_ENTRY_ACTIME
 	ktime_t ac_time;
 #endif