[PATCH 4.9 080/172] Btrfs: Fix deadlock between direct IO and fast fsync

Greg Kroah-Hartman
2017-07-03T13:43:24Z

From: Chandan Rajendra <chandan@linux.vnet.ibm.com>

[ 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.

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/btrfs/inode.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -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)