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Date: Tue, 1 Nov 2011 10:51:46 +0800
From: Tao Ma <taoma.tm@...il.com>
To: "Theodore Ts'o" <tytso@....edu>
Cc: Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 3/3] ext4: optimize locking for end_io extent conversion
Hi Ted,
On Mon, Oct 31, 2011 at 11:02 PM, Theodore Ts'o <tytso@....edu> wrote:
> Now that we are doing the locking correctly, we need to grab the
> i_completed_io_lock() twice per end_io. We can clean this up by
> removing the structure from the i_complted_io_list, and use this as
> the locking mechanism to prevent ext4_flush_completed_IO() racing
> against ext4_end_io_work(), instead of clearing the
> EXT4_IO_END_UNWRITTEN in io->flag.
>
> In addition, if the ext4_convert_unwritten_extents() returns an error,
> we no longer keep the end_io structure on the linked list. This
> doesn't help, because it tends to lock up the file system and wedges
> the system. That's one way to call attention to the problem, but it
> doesn't help the overall robustness of the system.
>
> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> ---
> fs/ext4/fsync.c | 5 ++---
> fs/ext4/page-io.c | 37 +++++++++++--------------------------
> 2 files changed, 13 insertions(+), 29 deletions(-)
>
> diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
> index 851ac5b..00a2cb7 100644
> --- a/fs/ext4/fsync.c
> +++ b/fs/ext4/fsync.c
> @@ -88,6 +88,7 @@ int ext4_flush_completed_IO(struct inode *inode)
> while (!list_empty(&ei->i_completed_io_list)){
> io = list_entry(ei->i_completed_io_list.next,
> ext4_io_end_t, list);
> + list_del_init(&io->list);
> /*
> * Calling ext4_end_io_nolock() to convert completed
> * IO to written.
> @@ -104,11 +105,9 @@ int ext4_flush_completed_IO(struct inode *inode)
> */
> spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> ret = ext4_end_io_nolock(io);
> - spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> if (ret < 0)
> ret2 = ret;
> - else
> - list_del_init(&io->list);
> + spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> }
> spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> return (ret2 < 0) ? ret2 : 0;
> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> index 4fa1d70..7bacd27 100644
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -99,28 +99,21 @@ int ext4_end_io_nolock(ext4_io_end_t *io)
> "list->prev 0x%p\n",
> io, inode->i_ino, io->list.next, io->list.prev);
>
> - if (!(io->flag & EXT4_IO_END_UNWRITTEN))
> - return ret;
> -
> ret = ext4_convert_unwritten_extents(inode, offset, size);
> if (ret < 0) {
> - printk(KERN_EMERG "%s: failed to convert unwritten "
> - "extents to written extents, error is %d "
> - "io is still on inode %lu aio dio list\n",
> - __func__, ret, inode->i_ino);
> - return ret;
> + ext4_msg(inode->i_sb, KERN_EMERG,
> + "failed to convert unwritten extents to written "
> + "extents -- potential data loss! "
> + "(inode %lu, offset %llu, size %d, error %d)",
> + inode->i_ino, offset, size, ret);
> }
>
> if (io->iocb)
> aio_complete(io->iocb, io->result, 0);
> - /* clear the DIO AIO unwritten flag */
> - if (io->flag & EXT4_IO_END_UNWRITTEN) {
> - io->flag &= ~EXT4_IO_END_UNWRITTEN;
> - /* Wake up anyone waiting on unwritten extent conversion */
> - if (atomic_dec_and_test(&EXT4_I(inode)->i_aiodio_unwritten))
> - wake_up_all(ext4_ioend_wq(io->inode));
> - }
>
> + /* Wake up anyone waiting on unwritten extent conversion */
> + if (atomic_dec_and_test(&EXT4_I(inode)->i_aiodio_unwritten))
> + wake_up_all(ext4_ioend_wq(io->inode));
> return ret;
> }
>
> @@ -133,16 +126,15 @@ static void ext4_end_io_work(struct work_struct *work)
> struct inode *inode = io->inode;
> struct ext4_inode_info *ei = EXT4_I(inode);
> unsigned long flags;
> - int ret;
>
> spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> if (list_empty(&io->list)) {
> spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> goto free;
> }
> - spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
>
> if (!mutex_trylock(&inode->i_mutex)) {
So here, we spin_lock first and then mutex_lock.
But in ext4_flush_completed_IO, we mutex_lock first and
then spin_lock. Will lockdep complain about it?
Other than that, the patch looks good to me.
Thanks
Tao
> + spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> /*
> * Requeue the work instead of waiting so that the work
> * items queued after this can be processed.
> @@ -159,16 +151,9 @@ static void ext4_end_io_work(struct work_struct *work)
> io->flag |= EXT4_IO_END_QUEUED;
> return;
> }
> - ret = ext4_end_io_nolock(io);
> - if (ret < 0) {
> - mutex_unlock(&inode->i_mutex);
> - return;
> - }
> -
> - spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> - if (!list_empty(&io->list))
> - list_del_init(&io->list);
> + list_del_init(&io->list);
> spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> + (void) ext4_end_io_nolock(io);
> mutex_unlock(&inode->i_mutex);
> free:
> ext4_free_io_end(io);
> --
> 1.7.4.1.22.gec8e1.dirty
>
> --
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