Patch "Btrfs: Fix deadlock between direct IO and fast fsync" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

Btrfs: Fix deadlock between direct IO and fast fsync

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
btrfs-fix-deadlock-between-direct-io-and-fast-fsync.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Chandan Rajendra <[email protected]>
Date: Fri, 23 Dec 2016 15:00:18 +0530
Subject: Btrfs: Fix deadlock between direct IO and fast fsync

From: Chandan Rajendra <[email protected]>

[ Upstream commit 97dcdea076ecef41ea4aaa23d4397c2f622e4265 ]

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.

---
 fs/btrfs/inode.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -7235,7 +7235,6 @@ static struct extent_map *btrfs_create_d
 	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,

@@ -7254,7 +7253,6 @@ static struct extent_map *btrfs_create_d
 		em = ERR_PTR(ret);
 	}
  out:
-	up_read(&BTRFS_I(inode)->dio_sem);
 
 	return em;
 }

@@ -8707,6 +8705,7 @@ static ssize_t btrfs_direct_IO(struct ki
 		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);

@@ -8719,6 +8718,7 @@ static ssize_t btrfs_direct_IO(struct ki
 				   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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/btrfs-fix-deadlock-between-direct-io-and-fast-fsync.patch