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Subject[PATCH] ubifs: Fix deadlock in concurrent bulk-read and writepage
Date
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@huawei.com>
Cc: <Stable@vger.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;
--
2.7.4
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