Re: [PATCH] f2fs: fix a deadlock in fsync

[Jim Xu]

Hi Kim,

The patch sounds good to me.

BTW, as in the deadlock situation I noticed, I did not find any thread was doing checkpoint at that moment. The bdi writeback thread and the fsync user thread was not. Neither was the background f2fs_gc thread because there had enough free segments. Meanwhile, another user thread was blocked in the f2fs_sync_file->sync_node_pages, which, I think, is not a factor contributing to the deadlock.

Thanks,
Jin

On 2013-8-6, at 20:46, Jaegeuk Kim <jaegeuk.kim@xxxxxxxxxxx> wrote:

> Hi, Jin,
> 
> IMO, this patch tries to fix the deadlock condition on
> f2fs_write_data_pages.
> 
> I think the errorneous scenario is something like this.
> When there remains only one fs_lock during the checkpoint procedure,
> f2fs_write_data_pages successfully gets the last one at the moment.
> Then, other operations like sync and writeback thread are definitely
> blocked too.
> 
> Meanwhile, in the flow of f2fs_write_data_pages, it is able to wait on
> writebacked node page, which is what you decribed.
> 
> If you indicated this scenario correctly, as I examined the flow again,
> I found one more case, __set_data_blkaddr, in addition to the
> update_inode. And, I can clean up another minor flow too.
> 
> Please check the below patch.
> Thanks,
> 
> ----> 
> 
> From a9c62162ea89c9b6b52d39d6db3f8f27c4d2ce5c Mon Sep 17 00:00:00 2001
> From: Jin Xu <jinuxstyle@xxxxxxxxx>
> Date: Mon, 5 Aug 2013 20:02:04 +0800
> Subject: [PATCH] f2fs: fix a deadlock in fsync
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx,
> linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> 
> This patch fixes a deadlock bug that occurs quite often when there are
> concurrent write and fsync on a same file.
> 
> Following is the simplified call trace when tasks get hung.
> 
> fsync thread:
> - f2fs_sync_file
> ...
> - f2fs_write_data_pages
> ...
>  - update_extent_cache
>  ...
>   - update_inode
>    - wait_on_page_writeback
> 
> bdi writeback thread
> - __writeback_single_inode
> - f2fs_write_data_pages
>  - mutex_lock(sbi->writepages)
> 
> The deadlock happens when the fsync thread waits on a inode page that
> has
> been added to the f2fs' cached bio sbi->bio[NODE], and unfortunately,
> no one else could be able to submit the cached bio to block layer for
> writeback. This is because the fsync thread already hold a sbi->fs_lock
> and
> the sbi->writepages lock, causing the bdi thread being blocked when
> attempt
> to write data pages for the same inode. At the same time, f2fs_gc thread
> does not notice the situation and could not help. Even the sync syscall
> gets blocked.
> 
> To fix it, we could submit the cached bio first before waiting on a
> inode page
> that is being written back.
> 
> Signed-off-by: Jin Xu <jinuxstyle@xxxxxxxxx>
> [Jaegeuk Kim: add more cases to use f2fs_wait_on_page_writeback]
> Signed-off-by: Jaegeuk Kim <jaegeuk.kim@xxxxxxxxxxx>
> ---
> fs/f2fs/data.c    |  2 +-
> fs/f2fs/f2fs.h    |  3 ++-
> fs/f2fs/gc.c      |  8 ++------
> fs/f2fs/inode.c   |  2 +-
> fs/f2fs/segment.c | 10 ++++++++++
> 5 files changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index f458883..a7eb529 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -37,7 +37,7 @@ static void __set_data_blkaddr(struct dnode_of_data
> *dn, block_t new_addr)
>    struct page *node_page = dn->node_page;
>    unsigned int ofs_in_node = dn->ofs_in_node;
> 
> -    wait_on_page_writeback(node_page);
> +    f2fs_wait_on_page_writeback(node_page, NODE, false);
> 
>    rn = F2FS_NODE(node_page);
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 63813be..13db10b 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -1023,7 +1023,8 @@ int npages_for_summary_flush(struct f2fs_sb_info
> *);
> void allocate_new_segments(struct f2fs_sb_info *);
> struct page *get_sum_page(struct f2fs_sb_info *, unsigned int);
> struct bio *f2fs_bio_alloc(struct block_device *, int);
> -void f2fs_submit_bio(struct f2fs_sb_info *, enum page_type, bool sync);
> +void f2fs_submit_bio(struct f2fs_sb_info *, enum page_type, bool);
> +void f2fs_wait_on_page_writeback(struct page *, enum page_type, bool);
> void write_meta_page(struct f2fs_sb_info *, struct page *);
> void write_node_page(struct f2fs_sb_info *, struct page *, unsigned
> int,
>                    block_t, block_t *);
> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> index d286d8b..e6b3ffd 100644
> --- a/fs/f2fs/gc.c
> +++ b/fs/f2fs/gc.c
> @@ -422,8 +422,7 @@ next_step:
> 
>        /* set page dirty and write it */
>        if (gc_type == FG_GC) {
> -            f2fs_submit_bio(sbi, NODE, true);
> -            wait_on_page_writeback(node_page);
> +            f2fs_wait_on_page_writeback(node_page, NODE, true);
>            set_page_dirty(node_page);
>        } else {
>            if (!PageWriteback(node_page))
> @@ -523,10 +522,7 @@ static void move_data_page(struct inode *inode,
> struct page *page, int gc_type)
>    } else {
>        struct f2fs_sb_info *sbi = F2FS_SB(inode->i_sb);
> 
> -        if (PageWriteback(page)) {
> -            f2fs_submit_bio(sbi, DATA, true);
> -            wait_on_page_writeback(page);
> -        }
> +        f2fs_wait_on_page_writeback(page, DATA, true);
> 
>        if (clear_page_dirty_for_io(page) &&
>            S_ISDIR(inode->i_mode)) {
> diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
> index debf743..9ab81e7 100644
> --- a/fs/f2fs/inode.c
> +++ b/fs/f2fs/inode.c
> @@ -151,7 +151,7 @@ void update_inode(struct inode *inode, struct page
> *node_page)
>    struct f2fs_node *rn;
>    struct f2fs_inode *ri;
> 
> -    wait_on_page_writeback(node_page);
> +    f2fs_wait_on_page_writeback(node_page, NODE, false);
> 
>    rn = F2FS_NODE(node_page);
>    ri = &(rn->i);
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 9b74ae2..68e344f 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -705,6 +705,16 @@ retry:
>    trace_f2fs_submit_write_page(page, blk_addr, type);
> }
> 
> +void f2fs_wait_on_page_writeback(struct page *page,
> +                enum page_type type, bool sync)
> +{
> +    struct f2fs_sb_info *sbi = F2FS_SB(page->mapping->host->i_sb);
> +    if (PageWriteback(page)) {
> +        f2fs_submit_bio(sbi, type, sync);
> +        wait_on_page_writeback(page);
> +    }
> +}
> +
> static bool __has_curseg_space(struct f2fs_sb_info *sbi, int type)
> {
>    struct curseg_info *curseg = CURSEG_I(sbi, type);
> -- 
> 1.8.3.1.437.g0dbd812
> 
> 
> 
> -- 
> Jaegeuk Kim
> Samsung
> 
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