writeback: fix time ordering of the per superblock dirty inode lists 6

[safe/jmp/linux-2.6] / fs / fs-writeback.c

/*
 * fs/fs-writeback.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * Contains all the functions related to writing back and waiting
 * upon dirty inodes against superblocks, and writing back dirty
 * pages against inodes.  ie: data writeback.  Writeout of the
 * inode itself is not handled here.
 *
 * 10Apr2002    akpm@zip.com.au
 *              Split out of fs/inode.c
 *              Additions for address_space-based writeback
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/spinlock.h>
#include <linux/sched.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/backing-dev.h>
#include <linux/buffer_head.h>
#include "internal.h"

/**
 *      __mark_inode_dirty -    internal function
 *      @inode: inode to mark
 *      @flags: what kind of dirty (i.e. I_DIRTY_SYNC)
 *      Mark an inode as dirty. Callers should use mark_inode_dirty or
 *      mark_inode_dirty_sync.
 *
 * Put the inode on the super block's dirty list.
 *
 * CAREFUL! We mark it dirty unconditionally, but move it onto the
 * dirty list only if it is hashed or if it refers to a blockdev.
 * If it was not hashed, it will never be added to the dirty list
 * even if it is later hashed, as it will have been marked dirty already.
 *
 * In short, make sure you hash any inodes _before_ you start marking
 * them dirty.
 *
 * This function *must* be atomic for the I_DIRTY_PAGES case -
 * set_page_dirty() is called under spinlock in several places.
 *
 * Note that for blockdevs, inode->dirtied_when represents the dirtying time of
 * the block-special inode (/dev/hda1) itself.  And the ->dirtied_when field of
 * the kernel-internal blockdev inode represents the dirtying time of the
 * blockdev's pages.  This is why for I_DIRTY_PAGES we always use
 * page->mapping->host, so the page-dirtying time is recorded in the internal
 * blockdev inode.
 */
void __mark_inode_dirty(struct inode *inode, int flags)
{
        struct super_block *sb = inode->i_sb;

        /*
         * Don't do this for I_DIRTY_PAGES - that doesn't actually
         * dirty the inode itself
         */
        if (flags & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
                if (sb->s_op->dirty_inode)
                        sb->s_op->dirty_inode(inode);
        }

        /*
         * make sure that changes are seen by all cpus before we test i_state
         * -- mikulas
         */
        smp_mb();

        /* avoid the locking if we can */
        if ((inode->i_state & flags) == flags)
                return;

        if (unlikely(block_dump)) {
                struct dentry *dentry = NULL;
                const char *name = "?";

                if (!list_empty(&inode->i_dentry)) {
                        dentry = list_entry(inode->i_dentry.next,
                                            struct dentry, d_alias);
                        if (dentry && dentry->d_name.name)
                                name = (const char *) dentry->d_name.name;
                }

                if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev"))
                        printk(KERN_DEBUG
                               "%s(%d): dirtied inode %lu (%s) on %s\n",
                               current->comm, current->pid, inode->i_ino,
                               name, inode->i_sb->s_id);
        }

        spin_lock(&inode_lock);
        if ((inode->i_state & flags) != flags) {
                const int was_dirty = inode->i_state & I_DIRTY;

                inode->i_state |= flags;

                /*
                 * If the inode is locked, just update its dirty state. 
                 * The unlocker will place the inode on the appropriate
                 * superblock list, based upon its state.
                 */
                if (inode->i_state & I_LOCK)
                        goto out;

                /*
                 * Only add valid (hashed) inodes to the superblock's
                 * dirty list.  Add blockdev inodes as well.
                 */
                if (!S_ISBLK(inode->i_mode)) {
                        if (hlist_unhashed(&inode->i_hash))
                                goto out;
                }
                if (inode->i_state & (I_FREEING|I_CLEAR))
                        goto out;

                /*
                 * If the inode was already on s_dirty or s_io, don't
                 * reposition it (that would break s_dirty time-ordering).
                 */
                if (!was_dirty) {
                        inode->dirtied_when = jiffies;
                        list_move(&inode->i_list, &sb->s_dirty);
                }
        }
out:
        spin_unlock(&inode_lock);
}

EXPORT_SYMBOL(__mark_inode_dirty);

static int write_inode(struct inode *inode, int sync)
{
        if (inode->i_sb->s_op->write_inode && !is_bad_inode(inode))
                return inode->i_sb->s_op->write_inode(inode, sync);
        return 0;
}

/*
 * Redirty an inode: set its when-it-was dirtied timestamp and move it to the
 * furthest end of its superblock's dirty-inode list.
 *
 * Before stamping the inode's ->dirtied_when, we check to see whether it is
 * already the most-recently-dirtied inode on the s_dirty list.  If that is
 * 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)
{
        struct super_block *sb = inode->i_sb;

        if (!list_empty(&sb->s_dirty)) {
                struct inode *tail_inode;

                tail_inode = list_entry(sb->s_dirty.next, struct inode, i_list);
                if (!time_after_eq(inode->dirtied_when,
                                tail_inode->dirtied_when))
                        inode->dirtied_when = jiffies;
        }
        list_move(&inode->i_list, &sb->s_dirty);
}

/*
 * Redirty an inode, but mark it as the very next-to-be-written inode on its
 * superblock's dirty-inode list.
 * We need to preserve s_dirty's reverse-time-orderedness, so we cheat by
 * setting this inode's dirtied_when to the same value as that of the inode
 * which is presently head-of-list, if present head-of-list is newer than this
 * inode. (head-of-list is the least-recently-dirtied inode: the oldest one).
 */
static void redirty_head(struct inode *inode)
{
        struct super_block *sb = inode->i_sb;

        if (!list_empty(&sb->s_dirty)) {
                struct inode *head_inode;

                head_inode = list_entry(sb->s_dirty.prev, struct inode, i_list);
                if (time_after(inode->dirtied_when, head_inode->dirtied_when))
                        inode->dirtied_when = head_inode->dirtied_when;
        }
        list_move_tail(&inode->i_list, &sb->s_dirty);
}

/*
 * Write a single inode's dirty pages and inode data out to disk.
 * If `wait' is set, wait on the writeout.
 *
 * The whole writeout design is quite complex and fragile.  We want to avoid
 * starvation of particular inodes when others are being redirtied, prevent
 * livelocks, etc.
 *
 * Called under inode_lock.
 */
static int
__sync_single_inode(struct inode *inode, struct writeback_control *wbc)
{
        unsigned dirty;
        struct address_space *mapping = inode->i_mapping;
        int wait = wbc->sync_mode == WB_SYNC_ALL;
        int ret;

        BUG_ON(inode->i_state & I_LOCK);

        /* Set I_LOCK, reset I_DIRTY */
        dirty = inode->i_state & I_DIRTY;
        inode->i_state |= I_LOCK;
        inode->i_state &= ~I_DIRTY;

        spin_unlock(&inode_lock);

        ret = do_writepages(mapping, wbc);

        /* Don't write the inode if only I_DIRTY_PAGES was set */
        if (dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
                int err = write_inode(inode, wait);
                if (ret == 0)
                        ret = err;
        }

        if (wait) {
                int err = filemap_fdatawait(mapping);
                if (ret == 0)
                        ret = err;
        }

        spin_lock(&inode_lock);
        inode->i_state &= ~I_LOCK;
        if (!(inode->i_state & I_FREEING)) {
                if (!(inode->i_state & I_DIRTY) &&
                    mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
                        /*
                         * We didn't write back all the pages.  nfs_writepages()
                         * sometimes bales out without doing anything. Redirty
                         * the inode.  It is moved from s_io onto s_dirty.
                         */
                        /*
                         * akpm: if the caller was the kupdate function we put
                         * this inode at the head of s_dirty so it gets first
                         * consideration.  Otherwise, move it to the tail, for
                         * the reasons described there.  I'm not really sure
                         * how much sense this makes.  Presumably I had a good
                         * reasons for doing it this way, and I'd rather not
                         * muck with it at present.
                         */
                        if (wbc->for_kupdate) {
                                /*
                                 * For the kupdate function we leave the inode
                                 * at the head of sb_dirty so it will get more
                                 * writeout as soon as the queue becomes
                                 * uncongested.
                                 */
                                inode->i_state |= I_DIRTY_PAGES;
                                redirty_head(inode);
                        } else {
                                /*
                                 * Otherwise fully redirty the inode so that
                                 * other inodes on this superblock will get some
                                 * writeout.  Otherwise heavy writing to one
                                 * file would indefinitely suspend writeout of
                                 * all the other files.
                                 */
                                inode->i_state |= I_DIRTY_PAGES;
                                redirty_tail(inode);
                        }
                } else if (inode->i_state & I_DIRTY) {
                        /*
                         * Someone redirtied the inode while were writing back
                         * the pages.
                         */
                        redirty_tail(inode);
                } else if (atomic_read(&inode->i_count)) {
                        /*
                         * The inode is clean, inuse
                         */
                        list_move(&inode->i_list, &inode_in_use);
                } else {
                        /*
                         * The inode is clean, unused
                         */
                        list_move(&inode->i_list, &inode_unused);
                }
        }
        wake_up_inode(inode);
        return ret;
}

/*
 * Write out an inode's dirty pages.  Called under inode_lock.  Either the
 * caller has ref on the inode (either via __iget or via syscall against an fd)
 * or the inode has I_WILL_FREE set (via generic_forget_inode)
 */
static int
__writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
{
        wait_queue_head_t *wqh;

        if (!atomic_read(&inode->i_count))
                WARN_ON(!(inode->i_state & (I_WILL_FREE|I_FREEING)));
        else
                WARN_ON(inode->i_state & I_WILL_FREE);

        if ((wbc->sync_mode != WB_SYNC_ALL) && (inode->i_state & I_LOCK)) {
                struct address_space *mapping = inode->i_mapping;
                int ret;

                /*
                 * We're skipping this inode because it's locked, and we're not
                 * doing writeback-for-data-integrity.  Move it to the head of
                 * s_dirty so that writeback can proceed with the other inodes
                 * on s_io.  We'll have another go at writing back this inode
                 * when the s_dirty iodes get moved back onto s_io.
                 */
                redirty_head(inode);

                /*
                 * Even if we don't actually write the inode itself here,
                 * we can at least start some of the data writeout..
                 */
                spin_unlock(&inode_lock);
                ret = do_writepages(mapping, wbc);
                spin_lock(&inode_lock);
                return ret;
        }

        /*
         * It's a data-integrity sync.  We must wait.
         */
        if (inode->i_state & I_LOCK) {
                DEFINE_WAIT_BIT(wq, &inode->i_state, __I_LOCK);

                wqh = bit_waitqueue(&inode->i_state, __I_LOCK);
                do {
                        spin_unlock(&inode_lock);
                        __wait_on_bit(wqh, &wq, inode_wait,
                                                        TASK_UNINTERRUPTIBLE);
                        spin_lock(&inode_lock);
                } while (inode->i_state & I_LOCK);
        }
        return __sync_single_inode(inode, wbc);
}

/*
 * Write out a superblock's list of dirty inodes.  A wait will be performed
 * upon no inodes, all inodes or the final one, depending upon sync_mode.
 *
 * If older_than_this is non-NULL, then only write out inodes which
 * had their first dirtying at a time earlier than *older_than_this.
 *
 * If we're a pdlfush thread, then implement pdflush collision avoidance
 * against the entire list.
 *
 * WB_SYNC_HOLD is a hack for sys_sync(): reattach the inode to sb->s_dirty so
 * that it can be located for waiting on in __writeback_single_inode().
 *
 * Called under inode_lock.
 *
 * If `bdi' is non-zero then we're being asked to writeback a specific queue.
 * This function assumes that the blockdev superblock's inodes are backed by
 * a variety of queues, so all inodes are searched.  For other superblocks,
 * assume that all inodes are backed by the same queue.
 *
 * FIXME: this linear search could get expensive with many fileystems.  But
 * how to fix?  We need to go from an address_space to all inodes which share
 * a queue with that address_space.  (Easy: have a global "dirty superblocks"
 * list).
 *
 * The inodes to be written are parked on sb->s_io.  They are moved back onto
 * sb->s_dirty as they are selected for writing.  This way, none can be missed
 * on the writer throttling path, and we get decent balancing between many
 * throttled threads: we don't want them all piling up on __wait_on_inode.
 */
static void
sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
{
        const unsigned long start = jiffies;    /* livelock avoidance */

        if (!wbc->for_kupdate || list_empty(&sb->s_io))
                list_splice_init(&sb->s_dirty, &sb->s_io);

        while (!list_empty(&sb->s_io)) {
                struct inode *inode = list_entry(sb->s_io.prev,
                                                struct inode, i_list);
                struct address_space *mapping = inode->i_mapping;
                struct backing_dev_info *bdi = mapping->backing_dev_info;
                long pages_skipped;

                if (!bdi_cap_writeback_dirty(bdi)) {
                        redirty_tail(inode);
                        if (sb_is_blkdev_sb(sb)) {
                                /*
                                 * Dirty memory-backed blockdev: the ramdisk
                                 * driver does this.  Skip just this inode
                                 */
                                continue;
                        }
                        /*
                         * Dirty memory-backed inode against a filesystem other
                         * than the kernel-internal bdev filesystem.  Skip the
                         * entire superblock.
                         */
                        break;
                }

                if (wbc->nonblocking && bdi_write_congested(bdi)) {
                        wbc->encountered_congestion = 1;
                        if (!sb_is_blkdev_sb(sb))
                                break;          /* Skip a congested fs */
                        list_move(&inode->i_list, &sb->s_dirty);
                        continue;               /* Skip a congested blockdev */
                }

                if (wbc->bdi && bdi != wbc->bdi) {
                        if (!sb_is_blkdev_sb(sb))
                                break;          /* fs has the wrong queue */
                        list_move(&inode->i_list, &sb->s_dirty);
                        continue;               /* blockdev has wrong queue */
                }

                /* Was this inode dirtied after sync_sb_inodes was called? */
                if (time_after(inode->dirtied_when, start))
                        break;

                /* Was this inode dirtied too recently? */
                if (wbc->older_than_this && time_after(inode->dirtied_when,
                                                *wbc->older_than_this))
                        break;

                /* Is another pdflush already flushing this queue? */
                if (current_is_pdflush() && !writeback_acquire(bdi))
                        break;

                BUG_ON(inode->i_state & I_FREEING);
                __iget(inode);
                pages_skipped = wbc->pages_skipped;
                __writeback_single_inode(inode, wbc);
                if (wbc->sync_mode == WB_SYNC_HOLD) {
                        inode->dirtied_when = jiffies;
                        list_move(&inode->i_list, &sb->s_dirty);
                }
                if (current_is_pdflush())
                        writeback_release(bdi);
                if (wbc->pages_skipped != pages_skipped) {
                        /*
                         * writeback is not making progress due to locked
                         * buffers.  Skip this inode for now.
                         */
                        redirty_tail(inode);
                }
                spin_unlock(&inode_lock);
                iput(inode);
                cond_resched();
                spin_lock(&inode_lock);
                if (wbc->nr_to_write <= 0)
                        break;
        }
        return;         /* Leave any unwritten inodes on s_io */
}

/*
 * Start writeback of dirty pagecache data against all unlocked inodes.
 *
 * Note:
 * We don't need to grab a reference to superblock here. If it has non-empty
 * ->s_dirty it's hadn't been killed yet and kill_super() won't proceed
 * past sync_inodes_sb() until both the ->s_dirty and ->s_io lists are
 * empty. Since __sync_single_inode() regains inode_lock before it finally moves
 * inode from superblock lists we are OK.
 *
 * If `older_than_this' is non-zero then only flush inodes which have a
 * flushtime older than *older_than_this.
 *
 * If `bdi' is non-zero then we will scan the first inode against each
 * superblock until we find the matching ones.  One group will be the dirty
 * inodes against a filesystem.  Then when we hit the dummy blockdev superblock,
 * sync_sb_inodes will seekout the blockdev which matches `bdi'.  Maybe not
 * super-efficient but we're about to do a ton of I/O...
 */
void
writeback_inodes(struct writeback_control *wbc)
{
        struct super_block *sb;

        might_sleep();
        spin_lock(&sb_lock);
restart:
        sb = sb_entry(super_blocks.prev);
        for (; sb != sb_entry(&super_blocks); sb = sb_entry(sb->s_list.prev)) {
                if (!list_empty(&sb->s_dirty) || !list_empty(&sb->s_io)) {
                        /* we're making our own get_super here */
                        sb->s_count++;
                        spin_unlock(&sb_lock);
                        /*
                         * If we can't get the readlock, there's no sense in
                         * waiting around, most of the time the FS is going to
                         * be unmounted by the time it is released.
                         */
                        if (down_read_trylock(&sb->s_umount)) {
                                if (sb->s_root) {
                                        spin_lock(&inode_lock);
                                        sync_sb_inodes(sb, wbc);
                                        spin_unlock(&inode_lock);
                                }
                                up_read(&sb->s_umount);
                        }
                        spin_lock(&sb_lock);
                        if (__put_super_and_need_restart(sb))
                                goto restart;
                }
                if (wbc->nr_to_write <= 0)
                        break;
        }
        spin_unlock(&sb_lock);
}

/*
 * writeback and wait upon the filesystem's dirty inodes.  The caller will
 * do this in two passes - one to write, and one to wait.  WB_SYNC_HOLD is
 * used to park the written inodes on sb->s_dirty for the wait pass.
 *
 * A finite limit is set on the number of pages which will be written.
 * To prevent infinite livelock of sys_sync().
 *
 * We add in the number of potentially dirty inodes, because each inode write
 * can dirty pagecache in the underlying blockdev.
 */
void sync_inodes_sb(struct super_block *sb, int wait)
{
        struct writeback_control wbc = {
                .sync_mode      = wait ? WB_SYNC_ALL : WB_SYNC_HOLD,
                .range_start    = 0,
                .range_end      = LLONG_MAX,
        };
        unsigned long nr_dirty = global_page_state(NR_FILE_DIRTY);
        unsigned long nr_unstable = global_page_state(NR_UNSTABLE_NFS);

        wbc.nr_to_write = nr_dirty + nr_unstable +
                        (inodes_stat.nr_inodes - inodes_stat.nr_unused) +
                        nr_dirty + nr_unstable;
        wbc.nr_to_write += wbc.nr_to_write / 2;         /* Bit more for luck */
        spin_lock(&inode_lock);
        sync_sb_inodes(sb, &wbc);
        spin_unlock(&inode_lock);
}

/*
 * Rather lame livelock avoidance.
 */
static void set_sb_syncing(int val)
{
        struct super_block *sb;
        spin_lock(&sb_lock);
        sb = sb_entry(super_blocks.prev);
        for (; sb != sb_entry(&super_blocks); sb = sb_entry(sb->s_list.prev)) {
                sb->s_syncing = val;
        }
        spin_unlock(&sb_lock);
}

/**
 * sync_inodes - writes all inodes to disk
 * @wait: wait for completion
 *
 * sync_inodes() goes through each super block's dirty inode list, writes the
 * inodes out, waits on the writeout and puts the inodes back on the normal
 * list.
 *
 * This is for sys_sync().  fsync_dev() uses the same algorithm.  The subtle
 * part of the sync functions is that the blockdev "superblock" is processed
 * last.  This is because the write_inode() function of a typical fs will
 * perform no I/O, but will mark buffers in the blockdev mapping as dirty.
 * What we want to do is to perform all that dirtying first, and then write
 * back all those inode blocks via the blockdev mapping in one sweep.  So the
 * additional (somewhat redundant) sync_blockdev() calls here are to make
 * sure that really happens.  Because if we call sync_inodes_sb(wait=1) with
 * outstanding dirty inodes, the writeback goes block-at-a-time within the
 * filesystem's write_inode().  This is extremely slow.
 */
static void __sync_inodes(int wait)
{
        struct super_block *sb;

        spin_lock(&sb_lock);
restart:
        list_for_each_entry(sb, &super_blocks, s_list) {
                if (sb->s_syncing)
                        continue;
                sb->s_syncing = 1;
                sb->s_count++;
                spin_unlock(&sb_lock);
                down_read(&sb->s_umount);
                if (sb->s_root) {
                        sync_inodes_sb(sb, wait);
                        sync_blockdev(sb->s_bdev);
                }
                up_read(&sb->s_umount);
                spin_lock(&sb_lock);
                if (__put_super_and_need_restart(sb))
                        goto restart;
        }
        spin_unlock(&sb_lock);
}

void sync_inodes(int wait)
{
        set_sb_syncing(0);
        __sync_inodes(0);

        if (wait) {
                set_sb_syncing(0);
                __sync_inodes(1);
        }
}

/**
 * write_inode_now      -       write an inode to disk
 * @inode: inode to write to disk
 * @sync: whether the write should be synchronous or not
 *
 * This function commits an inode to disk immediately if it is dirty. This is
 * primarily needed by knfsd.
 *
 * The caller must either have a ref on the inode or must have set I_WILL_FREE.
 */
int write_inode_now(struct inode *inode, int sync)
{
        int ret;
        struct writeback_control wbc = {
                .nr_to_write = LONG_MAX,
                .sync_mode = WB_SYNC_ALL,
                .range_start = 0,
                .range_end = LLONG_MAX,
        };

        if (!mapping_cap_writeback_dirty(inode->i_mapping))
                wbc.nr_to_write = 0;

        might_sleep();
        spin_lock(&inode_lock);
        ret = __writeback_single_inode(inode, &wbc);
        spin_unlock(&inode_lock);
        if (sync)
                wait_on_inode(inode);
        return ret;
}
EXPORT_SYMBOL(write_inode_now);

/**
 * sync_inode - write an inode and its pages to disk.
 * @inode: the inode to sync
 * @wbc: controls the writeback mode
 *
 * sync_inode() will write an inode and its pages to disk.  It will also
 * correctly update the inode on its superblock's dirty inode lists and will
 * update inode->i_state.
 *
 * The caller must have a ref on the inode.
 */
int sync_inode(struct inode *inode, struct writeback_control *wbc)
{
        int ret;

        spin_lock(&inode_lock);
        ret = __writeback_single_inode(inode, wbc);
        spin_unlock(&inode_lock);
        return ret;
}
EXPORT_SYMBOL(sync_inode);

/**
 * generic_osync_inode - flush all dirty data for a given inode to disk
 * @inode: inode to write
 * @mapping: the address_space that should be flushed
 * @what:  what to write and wait upon
 *
 * This can be called by file_write functions for files which have the
 * O_SYNC flag set, to flush dirty writes to disk.
 *
 * @what is a bitmask, specifying which part of the inode's data should be
 * written and waited upon.
 *
 *    OSYNC_DATA:     i_mapping's dirty data
 *    OSYNC_METADATA: the buffers at i_mapping->private_list
 *    OSYNC_INODE:    the inode itself
 */

int generic_osync_inode(struct inode *inode, struct address_space *mapping, int what)
{
        int err = 0;
        int need_write_inode_now = 0;
        int err2;

        if (what & OSYNC_DATA)
                err = filemap_fdatawrite(mapping);
        if (what & (OSYNC_METADATA|OSYNC_DATA)) {
                err2 = sync_mapping_buffers(mapping);
                if (!err)
                        err = err2;
        }
        if (what & OSYNC_DATA) {
                err2 = filemap_fdatawait(mapping);
                if (!err)
                        err = err2;
        }

        spin_lock(&inode_lock);
        if ((inode->i_state & I_DIRTY) &&
            ((what & OSYNC_INODE) || (inode->i_state & I_DIRTY_DATASYNC)))
                need_write_inode_now = 1;
        spin_unlock(&inode_lock);

        if (need_write_inode_now) {
                err2 = write_inode_now(inode, 1);
                if (!err)
                        err = err2;
        }
        else
                wait_on_inode(inode);

        return err;
}

EXPORT_SYMBOL(generic_osync_inode);

/**
 * writeback_acquire: attempt to get exclusive writeback access to a device
 * @bdi: the device's backing_dev_info structure
 *
 * It is a waste of resources to have more than one pdflush thread blocked on
 * a single request queue.  Exclusion at the request_queue level is obtained
 * via a flag in the request_queue's backing_dev_info.state.
 *
 * Non-request_queue-backed address_spaces will share default_backing_dev_info,
 * unless they implement their own.  Which is somewhat inefficient, as this
 * may prevent concurrent writeback against multiple devices.
 */
int writeback_acquire(struct backing_dev_info *bdi)
{
        return !test_and_set_bit(BDI_pdflush, &bdi->state);
}

/**
 * writeback_in_progress: determine whether there is writeback in progress
 * @bdi: the device's backing_dev_info structure.
 *
 * Determine whether there is writeback in progress against a backing device.
 */
int writeback_in_progress(struct backing_dev_info *bdi)
{
        return test_bit(BDI_pdflush, &bdi->state);
}

/**
 * writeback_release: relinquish exclusive writeback access against a device.
 * @bdi: the device's backing_dev_info structure
 */
void writeback_release(struct backing_dev_info *bdi)
{
        BUG_ON(!writeback_in_progress(bdi));
        clear_bit(BDI_pdflush, &bdi->state);
}