[PATCH] ubifs: Fix deadlock in concurrent bulk-read and writepage

[Zhihao Cheng <chengzhihao1@xxxxxxxxxx>]

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@xxxxxxxxxx>
Cc: <Stable@xxxxxxxxxxxxxxx>
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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