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writeback: flush inode cgroup wb switches instead of pinning super_block 

If cgroup writeback is in use, inodes can be scheduled for
asynchronous wb switching.  Before 5ff8eaac16 ("writeback: keep
superblock pinned during cgroup writeback association switches"), this
could race with umount leading to super_block being destroyed while
inodes are pinned for wb switching.  5ff8eaac16 fixed it by bumping
s_active while wb switches are in flight; however, this allowed
in-flight wb switches to make umounts asynchronous when the userland
expected synchronosity - e.g. fsck immediately following umount may
fail because the device is still busy.

This patch removes the problematic super_block pinning and instead
makes generic_shutdown_super() flush in-flight wb switches.  wb
switches are now executed on a dedicated isw_wq so that they can be
flushed and isw_nr_in_flight keeps track of the number of in-flight wb
switches so that flushing can be avoided in most cases.

v2: Move cgroup_writeback_umount() further below and add MS_ACTIVE
    check in inode_switch_wbs() as Jan an Al suggested.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Tahsin Erdogan <tahsin@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Link: http://lkml.kernel.org/g/CAAeU0aNCq7LGODvVGRU-oU_o-6enii5ey0p1c26D1ZzYwkDc5A@mail.gmail.com
Fixes: 5ff8eaac16 ("writeback: keep superblock pinned during cgroup writeback association switches")
Cc: stable@vger.kernel.org #v4.5
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Tejun Heo 2016-02-29 18:28:53 -05:00 committed by Jens Axboe
parent e9fc63d682
commit a1a0e23e49
3 changed files with 47 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

54
fs/fs-writeback.c
View File

@ -223,6 +223,9 @@ static void wb_wait_for_completion(struct backing_dev_info *bdi,
#define WB_FRN_HIST_MAX_SLOTS (WB_FRN_HIST_THR_SLOTS / 2 + 1)
/* one round can affect upto 5 slots */
static atomic_t isw_nr_in_flight = ATOMIC_INIT(0);
static struct workqueue_struct *isw_wq;
void __inode_attach_wb(struct inode *inode, struct page *page)
{
struct backing_dev_info *bdi = inode_to_bdi(inode);
@ -317,7 +320,6 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
struct inode_switch_wbs_context *isw =
container_of(work, struct inode_switch_wbs_context, work);
struct inode *inode = isw->inode;
struct super_block *sb = inode->i_sb;
struct address_space *mapping = inode->i_mapping;
struct bdi_writeback *old_wb = inode->i_wb;
struct bdi_writeback *new_wb = isw->new_wb;
@ -424,8 +426,9 @@ skip_switch:
wb_put(new_wb);
iput(inode);
deactivate_super(sb);
kfree(isw);
atomic_dec(&isw_nr_in_flight);
}
static void inode_switch_wbs_rcu_fn(struct rcu_head *rcu_head)
@ -435,7 +438,7 @@ static void inode_switch_wbs_rcu_fn(struct rcu_head *rcu_head)
/* needs to grab bh-unsafe locks, bounce to work item */
INIT_WORK(&isw->work, inode_switch_wbs_work_fn);
schedule_work(&isw->work);
queue_work(isw_wq, &isw->work);
}
/**
@ -471,20 +474,20 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
/* while holding I_WB_SWITCH, no one else can update the association */
spin_lock(&inode->i_lock);
if (inode->i_state & (I_WB_SWITCH | I_FREEING) ||
inode_to_wb(inode) == isw->new_wb)
goto out_unlock;
if (!atomic_inc_not_zero(&inode->i_sb->s_active))
goto out_unlock;
if (!(inode->i_sb->s_flags & MS_ACTIVE) ||
inode->i_state & (I_WB_SWITCH | I_FREEING) ||
inode_to_wb(inode) == isw->new_wb) {
spin_unlock(&inode->i_lock);
goto out_free;
}
inode->i_state |= I_WB_SWITCH;
spin_unlock(&inode->i_lock);
ihold(inode);
isw->inode = inode;
atomic_inc(&isw_nr_in_flight);
/*
* In addition to synchronizing among switchers, I_WB_SWITCH tells
* the RCU protected stat update paths to grab the mapping's
@ -494,8 +497,6 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
return;
out_unlock:
spin_unlock(&inode->i_lock);
out_free:
if (isw->new_wb)
wb_put(isw->new_wb);
@ -847,6 +848,33 @@ restart:
wb_put(last_wb);
}
/**
* cgroup_writeback_umount - flush inode wb switches for umount
*
* This function is called when a super_block is about to be destroyed and
* flushes in-flight inode wb switches. An inode wb switch goes through
* RCU and then workqueue, so the two need to be flushed in order to ensure
* that all previously scheduled switches are finished. As wb switches are
* rare occurrences and synchronize_rcu() can take a while, perform
* flushing iff wb switches are in flight.
*/
void cgroup_writeback_umount(void)
{
if (atomic_read(&isw_nr_in_flight)) {
synchronize_rcu();
flush_workqueue(isw_wq);
}
}
static int __init cgroup_writeback_init(void)
{
isw_wq = alloc_workqueue("inode_switch_wbs", 0, 0);
if (!isw_wq)
return -ENOMEM;
return 0;
}
fs_initcall(cgroup_writeback_init);
#else /* CONFIG_CGROUP_WRITEBACK */
static struct bdi_writeback *

1
fs/super.c
View File

@ -415,6 +415,7 @@ void generic_shutdown_super(struct super_block *sb)
sb->s_flags &= ~MS_ACTIVE;
fsnotify_unmount_inodes(sb);
cgroup_writeback_umount();
evict_inodes(sb);

5
include/linux/writeback.h
View File

@ -198,6 +198,7 @@ void wbc_attach_and_unlock_inode(struct writeback_control *wbc,
void wbc_detach_inode(struct writeback_control *wbc);
void wbc_account_io(struct writeback_control *wbc, struct page *page,
size_t bytes);
void cgroup_writeback_umount(void);
/**
* inode_attach_wb - associate an inode with its wb
@ -301,6 +302,10 @@ static inline void wbc_account_io(struct writeback_control *wbc,
{
}
static inline void cgroup_writeback_umount(void)
{
}
#endif /* CONFIG_CGROUP_WRITEBACK */
/*