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Date: Mon, 1 Oct 2012 20:38:33 +0200
From: Jan Kara <jack@...e.cz>
To: Dmitry Monakhov <dmonakhov@...nvz.org>
Cc: linux-ext4@...r.kernel.org, tytso@....edu, jack@...e.cz,
lczerner@...hat.com
Subject: Re: [PATCH 04/11] ext4: completed_io locking cleanup V4
On Fri 28-09-12 19:44:04, Dmitry Monakhov wrote:
Couple of language fixes:
> Current unwritten extent conversion state-machine is very fuzzy.
> - By unknown reason it want perform conversion under i_mutex. What for?
^^ For ^^^^^^^^ I'd just make it "performs"
> My diagnosis:
> We already protect extent tree with i_data_sem, truncate and punch_hole
> should wait for DIO, so the only data we have to protect is end_io->flags
> modification, but only flush_completed_IO and end_io_work modified this
^^ these
> flags and we can serialize them via i_completed_io_lock.
>
> Currently all this games with mutex_trylock result in following deadlock
^^^ these ^ the
> truncate: kworker:
> ext4_setattr ext4_end_io_work
> mutex_lock(i_mutex)
> inode_dio_wait(inode) ->BLOCK
> DEADLOCK<- mutex_trylock()
> inode_dio_done()
> #TEST_CASE1_BEGIN
> MNT=/mnt_scrach
> unlink $MNT/file
> fallocate -l $((1024*1024*1024)) $MNT/file
> aio-stress -I 100000 -O -s 100m -n -t 1 -c 10 -o 2 -o 3 $MNT/file
> sleep 2
> truncate -s 0 $MNT/file
> #TEST_CASE1_END
>
> Or use 286's xfstests https://github.com/dmonakhov/xfstests/blob/devel/286
>
> This patch makes state machine simple and clean:
>
> (1) xxx_end_io schedule final extent conversion simply by calling
> ext4_add_complete_io(), which append it to ei->i_completed_io_list
> NOTE1: because of (2A) work should be queued only if
> ->i_completed_io_list was empty, otherwise it work is scheduled already.
^^ remove this
>
> (2) ext4_flush_completed_IO is responsible for handling all pending
> end_io from ei->i_completed_io_list
> Flushing sequange consist of following stages:
^^ sequence ^ consists
> A) LOCKED: Atomically drain completed_io_list to local_list
> B) Perform extents conversion
> C) LOCKED: move conveted io's to to_free list for final deletion
^^ converted
> This logic depends on context which we was called from.
^^ were
> D) Final end_io context destruction
> NOTE1: i_mutex is no longer required because end_io->flags modification
> protected by ei->ext4_complete_io_lock
^^ is protected
>
> Full list of changes:
> - Move all completion end_io related routines to page-io.c in order to improve
> logic locality
> - Move open coded logic from various xx_end_xx routines to ext4_add_complete_io()
> - remove EXT4_IO_END_FSYNC
> - Improve SMP scalability by removing useless i_mutex which does not
> protect io->flags anymore.
> - Reduce lock contention on i_completed_io_lock by optimizing list walk.
> - Rename ext4_end_io_nolock to end4_end_io and make it static
> - Check flush completion status to ext4_ext_punch_hole(). Because it is
> not good idea to punch blocks from corrupted inode.
>
> Changes since V3 (in request to Jan's comments):
> Fall back to active flush_completed_IO() approach in order to prevent
> performance issues with nolocked DIO reads.
> Changes since V2:
> Fix use-after-free caused by race truncate vs end_io_work
>
> Signed-off-by: Dmitry Monakhov <dmonakhov@...nvz.org>
> ---
> fs/ext4/ext4.h | 3 +-
> fs/ext4/extents.c | 4 +-
> fs/ext4/fsync.c | 81 -------------------------
> fs/ext4/indirect.c | 6 +-
> fs/ext4/inode.c | 25 +-------
> fs/ext4/page-io.c | 171 ++++++++++++++++++++++++++++++++++------------------
> 6 files changed, 121 insertions(+), 169 deletions(-)
>
...
> +static int ext4_do_flush_completed_IO(struct inode *inode,
> + ext4_io_end_t *work_io)
> +{
> + ext4_io_end_t *io;
> + struct list_head unwritten, complete, to_free;
> + unsigned long flags;
> + struct ext4_inode_info *ei = EXT4_I(inode);
> + int err, ret = 0;
> +
> + INIT_LIST_HEAD(&complete);
> + INIT_LIST_HEAD(&to_free);
> +
> + spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> + dump_completed_IO(inode);
> + list_replace_init(&ei->i_completed_io_list, &unwritten);
> + spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> +
> + while (!list_empty(&unwritten)) {
> + io = list_entry(unwritten.next, ext4_io_end_t, list);
> + BUG_ON(!(io->flag & EXT4_IO_END_UNWRITTEN));
> + list_del_init(&io->list);
> +
> + err = ext4_end_io(io);
> + if (unlikely(!ret && err))
> + ret = err;
> +
> + list_add_tail(&io->list, &complete);
> + }
> + /* It is important to update all flags for all end_io in one shot w/o
> + * dropping the lock.*/
Why? It would seem that once io structures are moved from
i_completed_io_list, they are unreachable by anyone else?
> + spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> + while (!list_empty(&complete)) {
> + io = list_entry(complete.next, ext4_io_end_t, list);
> + io->flag &= ~EXT4_IO_END_UNWRITTEN;
> + /* end_io context can not be destroyed now because it still
> + * used by queued worker. Worker thread will destroy it later */
> + if (io->flag & EXT4_IO_END_QUEUED)
> + list_del_init(&io->list);
> + else
> + list_move(&io->list, &to_free);
> + }
> + /* If we are called from worker context, it is time to clear queued
> + * flag, and destroy it's end_io if it was converted already */
> + if (work_io) {
> + work_io->flag &= ~EXT4_IO_END_QUEUED;
> + if (!(work_io->flag & EXT4_IO_END_UNWRITTEN))
> + list_add_tail(&work_io->list, &to_free);
> }
> - 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);
> +
> + while (!list_empty(&to_free)) {
> + io = list_entry(to_free.next, ext4_io_end_t, list);
> + list_del_init(&io->list);
> + ext4_free_io_end(io);
> + }
> + return ret;
> +}
> +
> +/*
> + * work on completed aio dio IO, to convert unwritten extents to extents
> + */
> +static void ext4_end_io_work(struct work_struct *work)
> +{
> + ext4_io_end_t *io = container_of(work, ext4_io_end_t, work);
> + ext4_do_flush_completed_IO(io->inode, io);
> +}
> +
> +int ext4_flush_completed_IO(struct inode *inode)
> +{
> + return ext4_do_flush_completed_IO(inode, NULL);
> }
Also it seems that when ext4_flush_completed_IO() is called, workqueue
can have several IO structures queued in its local lists thus we miss them
here and don't properly wait for all conversions?
Honza
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