zram: Replace bit spinlocks with spinlock_t for PREEMPT_RT.

Commit Message

Sebastian Andrzej Siewior June 19, 2024, 3:08 p.m. UTC

From: Mike Galbraith <[e-mail]>

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 <[e-mail]>
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
---
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/[e-mail]
https://lore.kernel.org/[e-mail]

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

Comments

Jens Axboe June 19, 2024, 5:34 p.m. UTC | #1

On 6/19/24 9:08 AM, Sebastian Andrzej Siewior wrote:
> From: Mike Galbraith <[e-mail]>
> 
> 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.

The conditional code depending on CONFIG_PREEMPT_RT is nasty. Why not
just get rid of that and use the CONFIG_PREEMPT_RT variants for
everything? They are either good enough to work well in general, or it
should be redone such that it is.

Sebastian Andrzej Siewior June 19, 2024, 5:52 p.m. UTC | #2

On 2024-06-19 11:34:23 [-0600], Jens Axboe wrote:
> On 6/19/24 9:08 AM, Sebastian Andrzej Siewior wrote:
> > From: Mike Galbraith <[e-mail]>
> > 
> > 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.
> 
> The conditional code depending on CONFIG_PREEMPT_RT is nasty. Why not
> just get rid of that and use the CONFIG_PREEMPT_RT variants for
> everything? They are either good enough to work well in general, or it
> should be redone such that it is.

That would increase the struct size with lockdep for !RT. But it is
probably not a concern. Also other bits (besides ZRAM_LOCK) can not be
added but that wasn't needed in the last few years.
Okay, let me redo it.

Sebastian

Jens Axboe June 19, 2024, 6:01 p.m. UTC | #3

On 6/19/24 11:52 AM, Sebastian Andrzej Siewior wrote:
> On 2024-06-19 11:34:23 [-0600], Jens Axboe wrote:
>> On 6/19/24 9:08 AM, Sebastian Andrzej Siewior wrote:
>>> From: Mike Galbraith <[e-mail]>
>>>
>>> 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.
>>
>> The conditional code depending on CONFIG_PREEMPT_RT is nasty. Why not
>> just get rid of that and use the CONFIG_PREEMPT_RT variants for
>> everything? They are either good enough to work well in general, or it
>> should be redone such that it is.
> 
> That would increase the struct size with lockdep for !RT. But it is
> probably not a concern. Also other bits (besides ZRAM_LOCK) can not be
> added but that wasn't needed in the last few years.

Yeah I really don't think anyone cares about the struct size when
PROVE_LOCKING is on...

Patch

--- 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