Commit 97dcdea0 authored Dec 23, 2016 by Chandan Rajendra Committed by David Sterba Jan 19, 2017

Btrfs: Fix deadlock between direct IO and fast fsync



The following deadlock is seen when executing generic/113 test,

 ---------------------------------------------------------+----------------------------------------------------
  Direct I/O task                                           Fast fsync task
 ---------------------------------------------------------+----------------------------------------------------
  btrfs_direct_IO
    __blockdev_direct_IO
     do_blockdev_direct_IO
      do_direct_IO
       btrfs_get_blocks_direct
        while (blocks needs to written)
         get_more_blocks (first iteration)
          btrfs_get_blocks_direct
           btrfs_create_dio_extent
             down_read(&BTRFS_I(inode) >dio_sem)
             Create and add extent map and ordered extent
             up_read(&BTRFS_I(inode) >dio_sem)
                                                            btrfs_sync_file
                                                              btrfs_log_dentry_safe
                                                               btrfs_log_inode_parent
                                                                btrfs_log_inode
                                                                 btrfs_log_changed_extents
                                                                  down_write(&BTRFS_I(inode) >dio_sem)
                                                                   Collect new extent maps and ordered extents
                                                                    wait for ordered extent completion
         get_more_blocks (second iteration)
          btrfs_get_blocks_direct
           btrfs_create_dio_extent
             down_read(&BTRFS_I(inode) >dio_sem)
 --------------------------------------------------------------------------------------------------------------

In the above description, Btrfs direct I/O code path has not yet started
submitting bios for file range covered by the initial ordered
extent. Meanwhile, The fast fsync task obtains the write semaphore and
waits for I/O on the ordered extent to get completed. However, the
Direct I/O task is now blocked on obtaining the read semaphore.

To resolve the deadlock, this commit modifies the Direct I/O code path
to obtain the read semaphore before invoking
__blockdev_direct_IO(). The semaphore is then given up after
__blockdev_direct_IO() returns. This allows the Direct I/O code to
complete I/O on all the ordered extents it creates.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

parent 47b5d646

fs/btrfs/inode.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -7215,7 +7215,6 @@ static struct extent_map btrfs_create_dio_extent(struct inode inode,
		struct extent_map *em = NULL;
		int ret;

		down_read(&BTRFS_I(inode)->dio_sem);
		if (type != BTRFS_ORDERED_NOCOW) {
		em = create_pinned_em(inode, start, len, orig_start,
		block_start, block_len, orig_block_len,
		@@ -7234,7 +7233,6 @@ static struct extent_map btrfs_create_dio_extent(struct inode inode,
		em = ERR_PTR(ret);
		}
		out:
		up_read(&BTRFS_I(inode)->dio_sem);

		return em;
		}
		@@ -8695,6 +8693,7 @@ static ssize_t btrfs_direct_IO(struct kiocb iocb, struct iov_iter iter)
		dio_data.unsubmitted_oe_range_start = (u64)offset;
		dio_data.unsubmitted_oe_range_end = (u64)offset;
		current->journal_info = &dio_data;
		down_read(&BTRFS_I(inode)->dio_sem);
		} else if (test_bit(BTRFS_INODE_READDIO_NEED_LOCK,
		&BTRFS_I(inode)->runtime_flags)) {
		inode_dio_end(inode);
		@@ -8707,6 +8706,7 @@ static ssize_t btrfs_direct_IO(struct kiocb iocb, struct iov_iter iter)
		iter, btrfs_get_blocks_direct, NULL,
		btrfs_submit_direct, flags);
		if (iov_iter_rw(iter) == WRITE) {
		up_read(&BTRFS_I(inode)->dio_sem);
		current->journal_info = NULL;
		if (ret < 0 && ret != -EIOCBQUEUED) {
		if (dio_data.reserve)

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