Re: [PATCHES][RFC] packing struct block_device flags

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Apr 29, 2024 at 08:31:07AM +0100, Al Viro wrote:

> FWIW, we could go for atomic_t there and use
> 	atomic_read() & 0xff
> for partno, with atomic_or()/atomic_and() for set/clear and
> atomic_read() & constant for test.  That might slightly optimize
> set/clear on some architectures, but setting/clearing flags is
> nowhere near hot enough for that to make a difference.

Incremental for that (would be folded into 3/8 if we went that way)
is below; again, I'm not at all sure it's idiomatic enough to bother
with, but that should at least show what's going on:

diff --git a/block/bdev.c b/block/bdev.c
index 9aa23620fe92..fae30eae7741 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -411,7 +411,7 @@ struct block_device *bdev_alloc(struct gendisk *disk, u8 partno)
 	mutex_init(&bdev->bd_fsfreeze_mutex);
 	spin_lock_init(&bdev->bd_size_lock);
 	mutex_init(&bdev->bd_holder_lock);
-	bdev->__bd_flags = partno;
+	atomic_set(&bdev->__bd_flags, partno);
 	bdev->bd_inode = inode;
 	bdev->bd_queue = disk->queue;
 	if (partno && bdev_test_flag(disk->part0, BD_HAS_SUBMIT_BIO))
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 98e1c2d28d60..a822911e28e5 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -45,7 +45,7 @@ struct block_device {
 	struct request_queue *	bd_queue;
 	struct disk_stats __percpu *bd_stats;
 	unsigned long		bd_stamp;
-	u32			__bd_flags;	// partition number + flags
+	atomic_t		__bd_flags;	// partition number + flags
 	dev_t			bd_dev;
 	struct inode		*bd_inode;	/* will die */
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index d556cec9224b..a8271497ac62 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -722,38 +722,22 @@ void disk_uevent(struct gendisk *disk, enum kobject_action action);
 
 static inline u8 bdev_partno(const struct block_device *bdev)
 {
-	return bdev->__bd_flags & 0xff;
+	return atomic_read(&bdev->__bd_flags) & 0xff;
 }
 
 static inline bool bdev_test_flag(const struct block_device *bdev, int flag)
 {
-	return bdev->__bd_flags & (1 << (flag + 8));
+	return atomic_read(&bdev->__bd_flags) & (1 << (flag + 8));
 }
 
 static inline void bdev_set_flag(struct block_device *bdev, int flag)
 {
-	u32 v = bdev->__bd_flags;
-
-	for (;;) {
-		u32 w = cmpxchg(&bdev->__bd_flags, v, v | (1 << (flag + 8)));
-
-		if (v == w)
-			return;
-		v = w;
-	}
+	atomic_or(1 << (flag + 8), &bdev->__bd_flags);
 }
 
 static inline void bdev_clear_flag(struct block_device *bdev, int flag)
 {
-	u32 v = bdev->__bd_flags;
-
-	for (;;) {
-		u32 w = cmpxchg(&bdev->__bd_flags, v, v & ~(1 << (flag + 8)));
-
-		if (v == w)
-			return;
-		v = w;
-	}
+	atomic_and(~(1 << (flag + 8)), &bdev->__bd_flags);
 }
 
 static inline int get_disk_ro(struct gendisk *disk)