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Date: Sat, 11 Jan 2020 16:40:44 +0800
From: "zhangyi (F)" <yi.zhang@...wei.com>
To: Zhihao Cheng <chengzhihao1@...wei.com>, <richard@....at>,
<s.hauer@...gutronix.de>, <ext-adrian.hunter@...ia.com>
CC: <linux-mtd@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ubifs: Fix deadlock in concurrent bulk-read and writepage
Hi, Zhihao
Not sure the side effects of dropping ui->ui_mutex in ubifs_bulk_read(),
the inode->i_size may be incorrect due to the concurrent truncate?
I think it's better to pass FGP_NOWAIT when invoking pagecache_get_page()
and stop bulk read when we failed to lock the page, thoughts?
Yi.
On 2020/1/10 21:33, Zhihao Cheng wrote:
> In ubifs, concurrent execution of writepage and bulk read on the same file
> may cause ABBA deadlock, for example (Reproduce method see Link):
>
> Process A(Bulk-read starts from page4) Process B(write page4 back)
> vfs_read wb_workfn or fsync
> ... ...
> generic_file_buffered_read write_cache_pages
> ubifs_readpage LOCK(page4)
>
> ubifs_bulk_read ubifs_writepage
> LOCK(ui->ui_mutex) ubifs_write_inode
>
> ubifs_do_bulk_read LOCK(ui->ui_mutex)
> find_or_create_page(alloc page4) ↑
> LOCK(page4) <-- ABBA deadlock occurs!
>
> In order to ensure the serialization execution of bulk read, we can't
> remove the big lock 'ui->ui_mutex' in ubifs_bulk_read(). Instead, we add
> a new mutex lock for bulk read in ubifs_inode. ubifs_bulk_read() will be
> protected by the new mutex lock which replaces ui_mutex.
> It is confirmed that bulk-read (bulk read data member and process) and
> other members (which are protected by the ui_mutex in ubifs_inode) are
> independent of each other, including attr, dirty ("clean <-> dirty"
> transitions), ui_size (truncate process), etc.
>
> Signed-off-by: Zhihao Cheng <chengzhihao1@...wei.com>
> Cc: <Stable@...r.kernel.org>
> Fixes: 4793e7c5e1c ("UBIFS: add bulk-read facility")
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206153
> ---
> fs/ubifs/file.c | 21 +++++++++++++++++----
> fs/ubifs/super.c | 1 +
> fs/ubifs/ubifs.h | 7 +++++--
> 3 files changed, 23 insertions(+), 6 deletions(-)
>
> diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
> index cd52585..3e15c27 100644
> --- a/fs/ubifs/file.c
> +++ b/fs/ubifs/file.c
> @@ -836,10 +836,23 @@ static int ubifs_bulk_read(struct page *page)
> return 0;
>
> /*
> - * Bulk-read is protected by @ui->ui_mutex, but it is an optimization,
> - * so don't bother if we cannot lock the mutex.
> + * Bulk-read is protected by @ui->ui_bulk_read_mutex, ubifs_readpage()
> + * will all come here. If there are other concurrent read tasks during
> + * bulk-read, deadlock may occur, such as:
> + * Process A(Bulk-read starts from page4) Process B(Read page4)
> + * vfs_read vfs_read
> + * ... ...
> + * ubifs_readpage find_get_page(alloc page4)
> + * add_to_page_cache_lru
> + * ubifs_bulk_read LOCK(page4)
> + * LOCK(ui->ui_bulk_read_mutex) ubifs_readpage
> + * ubifs_do_bulk_read ubifs_bulk_read
> + * LOCK(ui->ui_bulk_read_mutex)
> + * find_or_create_page ↑
> + * LOCK(page4) <-- ABBA deadlock occurs!
> + * So don't bother if we cannot lock the mutex.
> */
> - if (!mutex_trylock(&ui->ui_mutex))
> + if (!mutex_trylock(&ui->ui_bulk_read_mutex))
> return 0;
>
> if (index != last_page_read + 1) {
> @@ -884,7 +897,7 @@ static int ubifs_bulk_read(struct page *page)
> kfree(bu);
>
> out_unlock:
> - mutex_unlock(&ui->ui_mutex);
> + mutex_unlock(&ui->ui_bulk_read_mutex);
> return err;
> }
>
> diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
> index 5e1e8ec..c4415b3 100644
> --- a/fs/ubifs/super.c
> +++ b/fs/ubifs/super.c
> @@ -257,6 +257,7 @@ static struct inode *ubifs_alloc_inode(struct super_block *sb)
> memset((void *)ui + sizeof(struct inode), 0,
> sizeof(struct ubifs_inode) - sizeof(struct inode));
> mutex_init(&ui->ui_mutex);
> + mutex_init(&ui->ui_bulk_read_mutex);
> spin_lock_init(&ui->ui_lock);
> return &ui->vfs_inode;
> };
> diff --git a/fs/ubifs/ubifs.h b/fs/ubifs/ubifs.h
> index c55f212..2ad596e 100644
> --- a/fs/ubifs/ubifs.h
> +++ b/fs/ubifs/ubifs.h
> @@ -354,8 +354,10 @@ struct ubifs_gced_idx_leb {
> * @xattr: non-zero if this is an extended attribute inode
> * @bulk_read: non-zero if bulk-read should be used
> * @ui_mutex: serializes inode write-back with the rest of VFS operations,
> - * serializes "clean <-> dirty" state changes, serializes bulk-read,
> - * protects @dirty, @bulk_read, @ui_size, and @xattr_size
> + * serializes "clean <-> dirty" state changes,
> + * protects @dirty, @ui_size, and @xattr_size
> + * @ui_bulk_read_mutex: serializes bulk-read, protects @bulk_read,
> + * @last_page_read and @read_in_a_row
> * @ui_lock: protects @synced_i_size
> * @synced_i_size: synchronized size of inode, i.e. the value of inode size
> * currently stored on the flash; used only for regular file
> @@ -409,6 +411,7 @@ struct ubifs_inode {
> unsigned int bulk_read:1;
> unsigned int compr_type:2;
> struct mutex ui_mutex;
> + struct mutex ui_bulk_read_mutex;
> spinlock_t ui_lock;
> loff_t synced_i_size;
> loff_t ui_size;
>
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