patches/fs-buffer-Make-BH_Uptodate_Lock-bit_spin_lock-a-regu.patch

From: Thomas Gleixner <tglx@linutronix.de>
Date: Fri, 15 Nov 2019 18:54:20 +0100
Subject: [PATCH] fs/buffer: Make BH_Uptodate_Lock bit_spin_lock a regular
 spinlock_t

Bit spinlocks are problematic if PREEMPT_RT is enabled, because they
disable preemption, which is undesired for latency reasons and breaks when
regular spinlocks are taken within the bit_spinlock locked region because
regular spinlocks are converted to 'sleeping spinlocks' on RT. So RT
replaces the bit spinlocks with regular spinlocks to avoid this problem.
Bit spinlocks are also not covered by lock debugging, e.g. lockdep.

Substitute the BH_Uptodate_Lock bit spinlock with a regular spinlock.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bigeasy: remove the wrapper and use always spinlock_t and move it into
          the padding hole]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
v2…v3: rename uptodate_lock to b_uptodate_lock.

v1…v2: Move the spinlock_t to the padding hole as per Jan Kara. pahole says
its total size remained unchanged, before

| atomic_t                   b_count;              /*    96     4 */
|
| /* size: 104, cachelines: 2, members: 12 */
| /* padding: 4 */
| /* last cacheline: 40 bytes */

after

| atomic_t                   b_count;              /*    96     4 */
| spinlock_t                 uptodate_lock;        /*   100     4 */
|
| /* size: 104, cachelines: 2, members: 13 */
| /* last cacheline: 40 bytes */

 fs/buffer.c                 |   19 +++++++------------
 fs/ext4/page-io.c           |    8 +++-----
 fs/ntfs/aops.c              |    9 +++------
 include/linux/buffer_head.h |    6 +++---
 4 files changed, 16 insertions(+), 26 deletions(-)

--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -274,8 +274,7 @@ static void end_buffer_async_read(struct
 	 * decide that the page is now completely done.
 	 */
 	first = page_buffers(page);
-	local_irq_save(flags);
-	bit_spin_lock(BH_Uptodate_Lock, &first->b_state);
+	spin_lock_irqsave(&first->b_uptodate_lock, flags);
 	clear_buffer_async_read(bh);
 	unlock_buffer(bh);
 	tmp = bh;
@@ -288,8 +287,7 @@ static void end_buffer_async_read(struct
 		}
 		tmp = tmp->b_this_page;
 	} while (tmp != bh);
-	bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&first->b_uptodate_lock, flags);
 
 	/*
 	 * If none of the buffers had errors and they are all
@@ -301,8 +299,7 @@ static void end_buffer_async_read(struct
 	return;
 
 still_busy:
-	bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&first->b_uptodate_lock, flags);
 	return;
 }
 
@@ -371,8 +368,7 @@ void end_buffer_async_write(struct buffe
 	}
 
 	first = page_buffers(page);
-	local_irq_save(flags);
-	bit_spin_lock(BH_Uptodate_Lock, &first->b_state);
+	spin_lock_irqsave(&first->b_uptodate_lock, flags);
 
 	clear_buffer_async_write(bh);
 	unlock_buffer(bh);
@@ -384,14 +380,12 @@ void end_buffer_async_write(struct buffe
 		}
 		tmp = tmp->b_this_page;
 	}
-	bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&first->b_uptodate_lock, flags);
 	end_page_writeback(page);
 	return;
 
 still_busy:
-	bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&first->b_uptodate_lock, flags);
 	return;
 }
 EXPORT_SYMBOL(end_buffer_async_write);
@@ -3396,6 +3390,7 @@ struct buffer_head *alloc_buffer_head(gf
 	struct buffer_head *ret = kmem_cache_zalloc(bh_cachep, gfp_flags);
 	if (ret) {
 		INIT_LIST_HEAD(&ret->b_assoc_buffers);
+		spin_lock_init(&ret->b_uptodate_lock);
 		preempt_disable();
 		__this_cpu_inc(bh_accounting.nr);
 		recalc_bh_state();
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -125,11 +125,10 @@ static void ext4_finish_bio(struct bio *
 		}
 		bh = head = page_buffers(page);
 		/*
-		 * We check all buffers in the page under BH_Uptodate_Lock
+		 * We check all buffers in the page under b_uptodate_lock
 		 * to avoid races with other end io clearing async_write flags
 		 */
-		local_irq_save(flags);
-		bit_spin_lock(BH_Uptodate_Lock, &head->b_state);
+		spin_lock_irqsave(&head->b_uptodate_lock, flags);
 		do {
 			if (bh_offset(bh) < bio_start ||
 			    bh_offset(bh) + bh->b_size > bio_end) {
@@ -141,8 +140,7 @@ static void ext4_finish_bio(struct bio *
 			if (bio->bi_status)
 				buffer_io_error(bh);
 		} while ((bh = bh->b_this_page) != head);
-		bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
-		local_irq_restore(flags);
+		spin_unlock_irqrestore(&head->b_uptodate_lock, flags);
 		if (!under_io) {
 			fscrypt_free_bounce_page(bounce_page);
 			end_page_writeback(page);
--- a/fs/ntfs/aops.c
+++ b/fs/ntfs/aops.c
@@ -92,8 +92,7 @@ static void ntfs_end_buffer_async_read(s
 				"0x%llx.", (unsigned long long)bh->b_blocknr);
 	}
 	first = page_buffers(page);
-	local_irq_save(flags);
-	bit_spin_lock(BH_Uptodate_Lock, &first->b_state);
+	spin_lock_irqsave(&first->b_uptodate_lock, flags);
 	clear_buffer_async_read(bh);
 	unlock_buffer(bh);
 	tmp = bh;
@@ -108,8 +107,7 @@ static void ntfs_end_buffer_async_read(s
 		}
 		tmp = tmp->b_this_page;
 	} while (tmp != bh);
-	bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&first->b_uptodate_lock, flags);
 	/*
 	 * If none of the buffers had errors then we can set the page uptodate,
 	 * but we first have to perform the post read mst fixups, if the
@@ -142,8 +140,7 @@ static void ntfs_end_buffer_async_read(s
 	unlock_page(page);
 	return;
 still_busy:
-	bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&first->b_uptodate_lock, flags);
 	return;
 }
 
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -22,9 +22,6 @@ enum bh_state_bits {
 	BH_Dirty,	/* Is dirty */
 	BH_Lock,	/* Is locked */
 	BH_Req,		/* Has been submitted for I/O */
-	BH_Uptodate_Lock,/* Used by the first bh in a page, to serialise
-			  * IO completion of other buffers in the page
-			  */
 
 	BH_Mapped,	/* Has a disk mapping */
 	BH_New,		/* Disk mapping was newly created by get_block */
@@ -76,6 +73,9 @@ struct buffer_head {
 	struct address_space *b_assoc_map;	/* mapping this buffer is
 						   associated with */
 	atomic_t b_count;		/* users using this buffer_head */
+	spinlock_t b_uptodate_lock;	/* Used by the first bh in a page, to
+					 * serialise IO completion of other
+					 * buffers in the page */
 };
 
 /*