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Subject[PATCH 07/15] writeback: split inode_wb_list_lock into bdi_writeback.list_lock
From: Christoph Hellwig <hch@infradead.org>

Split the global inode_wb_list_lock into a per-bdi_writeback list_lock,
as it's currently the most contended lock in the system for metadata
heavy workloads. It won't help for single-filesystem workloads for
which we'll need the I/O-less balance_dirty_pages, but at least we
can dedicate a cpu to spinning on each bdi now for larger systems.

Based on earlier patches from Nick Piggin and Dave Chinner.

It reduces lock contentions to 1/4 in this test case:
10 HDD JBOD, 100 dd on each disk, XFS, 6GB ram

lock_stat version 0.3
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total acq-bounces acquisitions holdtime-min holdtime-max holdtime-total
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
vanilla 2.6.39-rc3:
inode_wb_list_lock: 42590 44433 0.12 147.74 144127.35 252274 886792 0.08 121.34 917211.23
------------------
inode_wb_list_lock 2 [<ffffffff81165da5>] bdev_inode_switch_bdi+0x29/0x85
inode_wb_list_lock 34 [<ffffffff8115bd0b>] inode_wb_list_del+0x22/0x49
inode_wb_list_lock 12893 [<ffffffff8115bb53>] __mark_inode_dirty+0x170/0x1d0
inode_wb_list_lock 10702 [<ffffffff8115afef>] writeback_single_inode+0x16d/0x20a
------------------
inode_wb_list_lock 2 [<ffffffff81165da5>] bdev_inode_switch_bdi+0x29/0x85
inode_wb_list_lock 19 [<ffffffff8115bd0b>] inode_wb_list_del+0x22/0x49
inode_wb_list_lock 5550 [<ffffffff8115bb53>] __mark_inode_dirty+0x170/0x1d0
inode_wb_list_lock 8511 [<ffffffff8115b4ad>] writeback_sb_inodes+0x10f/0x157
2.6.39-rc3 + patch:
&(&wb->list_lock)->rlock: 11383 11657 0.14 151.69 40429.51 90825 527918 0.11 145.90 556843.37
------------------------
&(&wb->list_lock)->rlock 10 [<ffffffff8115b189>] inode_wb_list_del+0x5f/0x86
&(&wb->list_lock)->rlock 1493 [<ffffffff8115b1ed>] writeback_inodes_wb+0x3d/0x150
&(&wb->list_lock)->rlock 3652 [<ffffffff8115a8e9>] writeback_sb_inodes+0x123/0x16f
&(&wb->list_lock)->rlock 1412 [<ffffffff8115a38e>] writeback_single_inode+0x17f/0x223
------------------------
&(&wb->list_lock)->rlock 3 [<ffffffff8110b5af>] bdi_lock_two+0x46/0x4b
&(&wb->list_lock)->rlock 6 [<ffffffff8115b189>] inode_wb_list_del+0x5f/0x86
&(&wb->list_lock)->rlock 2061 [<ffffffff8115af97>] __mark_inode_dirty+0x173/0x1cf
&(&wb->list_lock)->rlock 2629 [<ffffffff8115a8e9>] writeback_sb_inodes+0x123/0x16f
hughd@google.com: fix recursive lock when bdi_lock_two() is called with new the same as old
akpm@linux-foundation.org: cleanup bdev_inode_switch_bdi() comment

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
fs/block_dev.c | 16 +++--
fs/fs-writeback.c | 97 +++++++++++++++++-----------------
fs/inode.c | 5 -
include/linux/backing-dev.h | 2
include/linux/writeback.h | 2
mm/backing-dev.c | 21 +++++--
mm/filemap.c | 6 +-
mm/rmap.c | 4 -
8 files changed, 85 insertions(+), 68 deletions(-)
--- linux-next.orig/fs/block_dev.c 2011-06-08 05:13:44.000000000 +0800
+++ linux-next/fs/block_dev.c 2011-06-08 05:14:32.000000000 +0800
@@ -44,24 +44,28 @@ inline struct block_device *I_BDEV(struc
{
return &BDEV_I(inode)->bdev;
}
-
EXPORT_SYMBOL(I_BDEV);

/*
- * move the inode from it's current bdi to the a new bdi. if the inode is dirty
- * we need to move it onto the dirty list of @dst so that the inode is always
- * on the right list.
+ * Move the inode from its current bdi to a new bdi. If the inode is dirty we
+ * need to move it onto the dirty list of @dst so that the inode is always on
+ * the right list.
*/
static void bdev_inode_switch_bdi(struct inode *inode,
struct backing_dev_info *dst)
{
- spin_lock(&inode_wb_list_lock);
+ struct backing_dev_info *old = inode->i_data.backing_dev_info;
+
+ if (unlikely(dst == old)) /* deadlock avoidance */
+ return;
+ bdi_lock_two(&old->wb, &dst->wb);
spin_lock(&inode->i_lock);
inode->i_data.backing_dev_info = dst;
if (inode->i_state & I_DIRTY)
list_move(&inode->i_wb_list, &dst->wb.b_dirty);
spin_unlock(&inode->i_lock);
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&old->wb.list_lock);
+ spin_unlock(&dst->wb.list_lock);
}

static sector_t max_block(struct block_device *bdev)
--- linux-next.orig/fs/fs-writeback.c 2011-06-08 05:14:14.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2011-06-08 05:14:32.000000000 +0800
@@ -181,12 +181,13 @@ void bdi_start_background_writeback(stru
*/
void inode_wb_list_del(struct inode *inode)
{
- spin_lock(&inode_wb_list_lock);
+ struct backing_dev_info *bdi = inode_to_bdi(inode);
+
+ spin_lock(&bdi->wb.list_lock);
list_del_init(&inode->i_wb_list);
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&bdi->wb.list_lock);
}

-
/*
* Redirty an inode: set its when-it-was dirtied timestamp and move it to the
* furthest end of its superblock's dirty-inode list.
@@ -196,11 +197,9 @@ void inode_wb_list_del(struct inode *ino
* the case then the inode must have been redirtied while it was being written
* out and we don't reset its dirtied_when.
*/
-static void redirty_tail(struct inode *inode)
+static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
{
- struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
-
- assert_spin_locked(&inode_wb_list_lock);
+ assert_spin_locked(&wb->list_lock);
if (!list_empty(&wb->b_dirty)) {
struct inode *tail;

@@ -214,11 +213,9 @@ static void redirty_tail(struct inode *i
/*
* requeue inode for re-scanning after bdi->b_io list is exhausted.
*/
-static void requeue_io(struct inode *inode)
+static void requeue_io(struct inode *inode, struct bdi_writeback *wb)
{
- struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
-
- assert_spin_locked(&inode_wb_list_lock);
+ assert_spin_locked(&wb->list_lock);
list_move(&inode->i_wb_list, &wb->b_more_io);
}

@@ -226,7 +223,7 @@ static void inode_sync_complete(struct i
{
/*
* Prevent speculative execution through
- * spin_unlock(&inode_wb_list_lock);
+ * spin_unlock(&wb->list_lock);
*/

smp_mb();
@@ -302,7 +299,7 @@ static void move_expired_inodes(struct l
*/
static void queue_io(struct bdi_writeback *wb, unsigned long *older_than_this)
{
- assert_spin_locked(&inode_wb_list_lock);
+ assert_spin_locked(&wb->list_lock);
list_splice_init(&wb->b_more_io, &wb->b_io);
move_expired_inodes(&wb->b_dirty, &wb->b_io, older_than_this);
}
@@ -317,7 +314,8 @@ static int write_inode(struct inode *ino
/*
* Wait for writeback on an inode to complete.
*/
-static void inode_wait_for_writeback(struct inode *inode)
+static void inode_wait_for_writeback(struct inode *inode,
+ struct bdi_writeback *wb)
{
DEFINE_WAIT_BIT(wq, &inode->i_state, __I_SYNC);
wait_queue_head_t *wqh;
@@ -325,15 +323,15 @@ static void inode_wait_for_writeback(str
wqh = bit_waitqueue(&inode->i_state, __I_SYNC);
while (inode->i_state & I_SYNC) {
spin_unlock(&inode->i_lock);
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&wb->list_lock);
__wait_on_bit(wqh, &wq, inode_wait, TASK_UNINTERRUPTIBLE);
- spin_lock(&inode_wb_list_lock);
+ spin_lock(&wb->list_lock);
spin_lock(&inode->i_lock);
}
}

/*
- * Write out an inode's dirty pages. Called under inode_wb_list_lock and
+ * Write out an inode's dirty pages. Called under wb->list_lock and
* inode->i_lock. Either the caller has an active reference on the inode or
* the inode has I_WILL_FREE set.
*
@@ -344,13 +342,14 @@ static void inode_wait_for_writeback(str
* livelocks, etc.
*/
static int
-writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
+writeback_single_inode(struct inode *inode, struct bdi_writeback *wb,
+ struct writeback_control *wbc)
{
struct address_space *mapping = inode->i_mapping;
unsigned dirty;
int ret;

- assert_spin_locked(&inode_wb_list_lock);
+ assert_spin_locked(&wb->list_lock);
assert_spin_locked(&inode->i_lock);

if (!atomic_read(&inode->i_count))
@@ -368,14 +367,14 @@ writeback_single_inode(struct inode *ino
* completed a full scan of b_io.
*/
if (wbc->sync_mode != WB_SYNC_ALL) {
- requeue_io(inode);
+ requeue_io(inode, wb);
return 0;
}

/*
* It's a data-integrity sync. We must wait.
*/
- inode_wait_for_writeback(inode);
+ inode_wait_for_writeback(inode, wb);
}

BUG_ON(inode->i_state & I_SYNC);
@@ -384,7 +383,7 @@ writeback_single_inode(struct inode *ino
inode->i_state |= I_SYNC;
inode->i_state &= ~I_DIRTY_PAGES;
spin_unlock(&inode->i_lock);
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&wb->list_lock);

ret = do_writepages(mapping, wbc);

@@ -415,7 +414,7 @@ writeback_single_inode(struct inode *ino
ret = err;
}

- spin_lock(&inode_wb_list_lock);
+ spin_lock(&wb->list_lock);
spin_lock(&inode->i_lock);
inode->i_state &= ~I_SYNC;
if (!(inode->i_state & I_FREEING)) {
@@ -438,7 +437,7 @@ writeback_single_inode(struct inode *ino
/*
* slice used up: queue for next turn
*/
- requeue_io(inode);
+ requeue_io(inode, wb);
} else {
/*
* Writeback blocked by something other than
@@ -447,7 +446,7 @@ writeback_single_inode(struct inode *ino
* retrying writeback of the dirty page/inode
* that cannot be performed immediately.
*/
- redirty_tail(inode);
+ redirty_tail(inode, wb);
}
} else if (inode->i_state & I_DIRTY) {
/*
@@ -456,7 +455,7 @@ writeback_single_inode(struct inode *ino
* submission or metadata updates after data IO
* completion.
*/
- redirty_tail(inode);
+ redirty_tail(inode, wb);
} else {
/*
* The inode is clean. At this point we either have
@@ -521,7 +520,7 @@ static int writeback_sb_inodes(struct su
* superblock, move all inodes not belonging
* to it back onto the dirty list.
*/
- redirty_tail(inode);
+ redirty_tail(inode, wb);
continue;
}

@@ -541,7 +540,7 @@ static int writeback_sb_inodes(struct su
spin_lock(&inode->i_lock);
if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) {
spin_unlock(&inode->i_lock);
- requeue_io(inode);
+ requeue_io(inode, wb);
continue;
}

@@ -557,19 +556,19 @@ static int writeback_sb_inodes(struct su
__iget(inode);

pages_skipped = wbc->pages_skipped;
- writeback_single_inode(inode, wbc);
+ writeback_single_inode(inode, wb, wbc);
if (wbc->pages_skipped != pages_skipped) {
/*
* writeback is not making progress due to locked
* buffers. Skip this inode for now.
*/
- redirty_tail(inode);
+ redirty_tail(inode, wb);
}
spin_unlock(&inode->i_lock);
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&wb->list_lock);
iput(inode);
cond_resched();
- spin_lock(&inode_wb_list_lock);
+ spin_lock(&wb->list_lock);
if (wbc->nr_to_write <= 0) {
wbc->more_io = 1;
return 1;
@@ -588,7 +587,7 @@ void writeback_inodes_wb(struct bdi_writ

if (!wbc->wb_start)
wbc->wb_start = jiffies; /* livelock avoidance */
- spin_lock(&inode_wb_list_lock);
+ spin_lock(&wb->list_lock);

if (list_empty(&wb->b_io))
queue_io(wb, wbc->older_than_this);
@@ -598,7 +597,7 @@ void writeback_inodes_wb(struct bdi_writ
struct super_block *sb = inode->i_sb;

if (!pin_sb_for_writeback(sb)) {
- requeue_io(inode);
+ requeue_io(inode, wb);
continue;
}
ret = writeback_sb_inodes(sb, wb, wbc, false);
@@ -607,7 +606,7 @@ void writeback_inodes_wb(struct bdi_writ
if (ret)
break;
}
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&wb->list_lock);
/* Leave any unwritten inodes on b_io */
}

@@ -616,11 +615,11 @@ static void __writeback_inodes_sb(struct
{
WARN_ON(!rwsem_is_locked(&sb->s_umount));

- spin_lock(&inode_wb_list_lock);
+ spin_lock(&wb->list_lock);
if (list_empty(&wb->b_io))
queue_io(wb, wbc->older_than_this);
writeback_sb_inodes(sb, wb, wbc, true);
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&wb->list_lock);
}

/*
@@ -762,15 +761,15 @@ static long wb_writeback(struct bdi_writ
* become available for writeback. Otherwise
* we'll just busyloop.
*/
- spin_lock(&inode_wb_list_lock);
+ spin_lock(&wb->list_lock);
if (!list_empty(&wb->b_more_io)) {
inode = wb_inode(wb->b_more_io.prev);
trace_wbc_writeback_wait(&wbc, wb->bdi);
spin_lock(&inode->i_lock);
- inode_wait_for_writeback(inode);
+ inode_wait_for_writeback(inode, wb);
spin_unlock(&inode->i_lock);
}
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&wb->list_lock);
}

return wrote;
@@ -1104,10 +1103,10 @@ void __mark_inode_dirty(struct inode *in
}

spin_unlock(&inode->i_lock);
- spin_lock(&inode_wb_list_lock);
+ spin_lock(&bdi->wb.list_lock);
inode->dirtied_when = jiffies;
list_move(&inode->i_wb_list, &bdi->wb.b_dirty);
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&bdi->wb.list_lock);

if (wakeup_bdi)
bdi_wakeup_thread_delayed(bdi);
@@ -1309,6 +1308,7 @@ EXPORT_SYMBOL(sync_inodes_sb);
*/
int write_inode_now(struct inode *inode, int sync)
{
+ struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
int ret;
struct writeback_control wbc = {
.nr_to_write = LONG_MAX,
@@ -1321,11 +1321,11 @@ int write_inode_now(struct inode *inode,
wbc.nr_to_write = 0;

might_sleep();
- spin_lock(&inode_wb_list_lock);
+ spin_lock(&wb->list_lock);
spin_lock(&inode->i_lock);
- ret = writeback_single_inode(inode, &wbc);
+ ret = writeback_single_inode(inode, wb, &wbc);
spin_unlock(&inode->i_lock);
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&wb->list_lock);
if (sync)
inode_sync_wait(inode);
return ret;
@@ -1345,13 +1345,14 @@ EXPORT_SYMBOL(write_inode_now);
*/
int sync_inode(struct inode *inode, struct writeback_control *wbc)
{
+ struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
int ret;

- spin_lock(&inode_wb_list_lock);
+ spin_lock(&wb->list_lock);
spin_lock(&inode->i_lock);
- ret = writeback_single_inode(inode, wbc);
+ ret = writeback_single_inode(inode, wb, wbc);
spin_unlock(&inode->i_lock);
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&wb->list_lock);
return ret;
}
EXPORT_SYMBOL(sync_inode);
--- linux-next.orig/fs/inode.c 2011-06-08 05:13:45.000000000 +0800
+++ linux-next/fs/inode.c 2011-06-08 05:14:32.000000000 +0800
@@ -37,7 +37,7 @@
* inode_lru, inode->i_lru
* inode_sb_list_lock protects:
* sb->s_inodes, inode->i_sb_list
- * inode_wb_list_lock protects:
+ * bdi->wb.list_lock protects:
* bdi->wb.b_{dirty,io,more_io}, inode->i_wb_list
* inode_hash_lock protects:
* inode_hashtable, inode->i_hash
@@ -48,7 +48,7 @@
* inode->i_lock
* inode_lru_lock
*
- * inode_wb_list_lock
+ * bdi->wb.list_lock
* inode->i_lock
*
* inode_hash_lock
@@ -68,7 +68,6 @@ static LIST_HEAD(inode_lru);
static DEFINE_SPINLOCK(inode_lru_lock);

__cacheline_aligned_in_smp DEFINE_SPINLOCK(inode_sb_list_lock);
-__cacheline_aligned_in_smp DEFINE_SPINLOCK(inode_wb_list_lock);

/*
* iprune_sem provides exclusion between the icache shrinking and the
--- linux-next.orig/include/linux/backing-dev.h 2011-06-08 05:11:57.000000000 +0800
+++ linux-next/include/linux/backing-dev.h 2011-06-08 05:14:32.000000000 +0800
@@ -57,6 +57,7 @@ struct bdi_writeback {
struct list_head b_dirty; /* dirty inodes */
struct list_head b_io; /* parked for writeback */
struct list_head b_more_io; /* parked for more writeback */
+ spinlock_t list_lock; /* protects the b_* lists */
};

struct backing_dev_info {
@@ -106,6 +107,7 @@ int bdi_writeback_thread(void *data);
int bdi_has_dirty_io(struct backing_dev_info *bdi);
void bdi_arm_supers_timer(void);
void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi);
+void bdi_lock_two(struct bdi_writeback *wb1, struct bdi_writeback *wb2);

extern spinlock_t bdi_lock;
extern struct list_head bdi_list;
--- linux-next.orig/include/linux/writeback.h 2011-06-08 05:14:14.000000000 +0800
+++ linux-next/include/linux/writeback.h 2011-06-08 05:14:32.000000000 +0800
@@ -9,8 +9,6 @@

struct backing_dev_info;

-extern spinlock_t inode_wb_list_lock;
-
/*
* fs/fs-writeback.c
*/
--- linux-next.orig/mm/backing-dev.c 2011-06-08 05:13:47.000000000 +0800
+++ linux-next/mm/backing-dev.c 2011-06-08 05:14:48.000000000 +0800
@@ -45,6 +45,17 @@ static struct timer_list sync_supers_tim
static int bdi_sync_supers(void *);
static void sync_supers_timer_fn(unsigned long);

+void bdi_lock_two(struct bdi_writeback *wb1, struct bdi_writeback *wb2)
+{
+ if (wb1 < wb2) {
+ spin_lock(&wb1->list_lock);
+ spin_lock_nested(&wb2->list_lock, 1);
+ } else {
+ spin_lock(&wb2->list_lock);
+ spin_lock_nested(&wb1->list_lock, 1);
+ }
+}
+
#ifdef CONFIG_DEBUG_FS
#include <linux/debugfs.h>
#include <linux/seq_file.h>
@@ -67,14 +78,14 @@ static int bdi_debug_stats_show(struct s
struct inode *inode;

nr_dirty = nr_io = nr_more_io = 0;
- spin_lock(&inode_wb_list_lock);
+ spin_lock(&wb->list_lock);
list_for_each_entry(inode, &wb->b_dirty, i_wb_list)
nr_dirty++;
list_for_each_entry(inode, &wb->b_io, i_wb_list)
nr_io++;
list_for_each_entry(inode, &wb->b_more_io, i_wb_list)
nr_more_io++;
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&wb->list_lock);

global_dirty_limits(&background_thresh, &dirty_thresh);
bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
@@ -628,6 +639,7 @@ static void bdi_wb_init(struct bdi_write
INIT_LIST_HEAD(&wb->b_dirty);
INIT_LIST_HEAD(&wb->b_io);
INIT_LIST_HEAD(&wb->b_more_io);
+ spin_lock_init(&wb->list_lock);
setup_timer(&wb->wakeup_timer, wakeup_timer_fn, (unsigned long)bdi);
}

@@ -676,11 +688,12 @@ void bdi_destroy(struct backing_dev_info
if (bdi_has_dirty_io(bdi)) {
struct bdi_writeback *dst = &default_backing_dev_info.wb;

- spin_lock(&inode_wb_list_lock);
+ bdi_lock_two(&bdi->wb, dst);
list_splice(&bdi->wb.b_dirty, &dst->b_dirty);
list_splice(&bdi->wb.b_io, &dst->b_io);
list_splice(&bdi->wb.b_more_io, &dst->b_more_io);
- spin_unlock(&inode_wb_list_lock);
+ spin_unlock(&bdi->wb.list_lock);
+ spin_unlock(&dst->list_lock);
}

bdi_unregister(bdi);
--- linux-next.orig/mm/filemap.c 2011-06-08 05:13:47.000000000 +0800
+++ linux-next/mm/filemap.c 2011-06-08 05:14:32.000000000 +0800
@@ -81,7 +81,7 @@
* ->i_mutex
* ->i_alloc_sem (various)
*
- * inode_wb_list_lock
+ * bdi->wb.list_lock
* sb_lock (fs/fs-writeback.c)
* ->mapping->tree_lock (__sync_single_inode)
*
@@ -99,9 +99,9 @@
* ->zone.lru_lock (check_pte_range->isolate_lru_page)
* ->private_lock (page_remove_rmap->set_page_dirty)
* ->tree_lock (page_remove_rmap->set_page_dirty)
- * inode_wb_list_lock (page_remove_rmap->set_page_dirty)
+ * bdi.wb->list_lock (page_remove_rmap->set_page_dirty)
* ->inode->i_lock (page_remove_rmap->set_page_dirty)
- * inode_wb_list_lock (zap_pte_range->set_page_dirty)
+ * bdi.wb->list_lock (zap_pte_range->set_page_dirty)
* ->inode->i_lock (zap_pte_range->set_page_dirty)
* ->private_lock (zap_pte_range->__set_page_dirty_buffers)
*
--- linux-next.orig/mm/rmap.c 2011-06-08 05:13:47.000000000 +0800
+++ linux-next/mm/rmap.c 2011-06-08 05:14:32.000000000 +0800
@@ -32,11 +32,11 @@
* mmlist_lock (in mmput, drain_mmlist and others)
* mapping->private_lock (in __set_page_dirty_buffers)
* inode->i_lock (in set_page_dirty's __mark_inode_dirty)
- * inode_wb_list_lock (in set_page_dirty's __mark_inode_dirty)
+ * bdi.wb->list_lock (in set_page_dirty's __mark_inode_dirty)
* sb_lock (within inode_lock in fs/fs-writeback.c)
* mapping->tree_lock (widely used, in set_page_dirty,
* in arch-dependent flush_dcache_mmap_lock,
- * within inode_wb_list_lock in __sync_single_inode)
+ * within bdi.wb->list_lock in __sync_single_inode)
*
* (code doesn't rely on that order so it could be switched around)
* ->tasklist_lock



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