btrfs: fix deadlock between concurrent dio writes when low on free data space

Summary

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix deadlock between concurrent dio writes when low on free data space

When reserving data space for a direct IO write we can end up deadlocking if we have multiple tasks attempting a write to the same file range, there are multiple extents covered by that file range, we are low on available space for data and the writes don't expand the inode's i_size.

The deadlock can happen like this:

1) We have a file with an i_size of 1M, at offset 0 it has an extent with a size of 128K and at offset 128K it has another extent also with a size of 128K;

2) Task A does a direct IO write against file range [0, 256K), and because the write is within the i_size boundary, it takes the inode's lock (VFS level) in shared mode;

3) Task A locks the file range [0, 256K) at btrfs_dio_iomap_begin(), and then gets the extent map for the extent covering the range [0, 128K). At btrfs_get_blocks_direct_write(), it creates an ordered extent for that file range ([0, 128K));

4) Before returning from btrfs_dio_iomap_begin(), it unlocks the file range [0, 256K);

5) Task A executes btrfs_dio_iomap_begin() again, this time for the file range [128K, 256K), and locks the file range [128K, 256K);

6) Task B starts a direct IO write against file range [0, 256K) as well. It also locks the inode in shared mode, as it's within the i_size limit, and then tries to lock file range [0, 256K). It is able to lock the subrange [0, 128K) but then blocks waiting for the range [128K, 256K), as it is currently locked by task A;

7) Task A enters btrfs_get_blocks_direct_write() and tries to reserve data space. Because we are low on available free space, it triggers the async data reclaim task, and waits for it to reserve data space;

8) The async reclaim task decides to wait for all existing ordered extents to complete (through btrfs_wait_ordered_roots()). It finds the ordered extent previously created by task A for the file range [0, 128K) and waits for it to complete;

9) The ordered extent for the file range [0, 128K) can not complete because it blocks at btrfs_finish_ordered_io() when trying to lock the file range [0, 128K).

This results in a deadlock, because:

- task B is holding the file range [0, 128K) locked, waiting for the range [128K, 256K) to be unlocked by task A;

- task A is holding the file range [128K, 256K) locked and it's waiting for the async data reclaim task to satisfy its space reservation request;

- the async data reclaim task is waiting for ordered extent [0, 128K) to complete, but the ordered extent can not complete because the file range [0, 128K) is currently locked by task B, which is waiting on task A to unlock file range [128K, 256K) and task A waiting on the async data reclaim task.

This results in a deadlock between 4 task: task A, task B, the async data reclaim task and the task doing ordered extent completion (a work queue task).

This type of deadlock can sporadically be triggered by the test case generic/300 from fstests, and results in a stack trace like the following:

[12084.033689] INFO: task kworker/u16:7:123749 blocked for more than 241 seconds. [12084.034877] Not tainted 5.18.0-rc2-btrfs-next-115 #1 [12084.035562] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [12084.036548] task:kworker/u16:7 state:D stack: 0 pid:123749 ppid: 2 flags:0x00004000 [12084.036554] Workqueue: btrfs-flush_delalloc btrfs_work_helper [btrfs] [12084.036599] Call Trace: [12084.036601] <TASK> [12084.036606] __schedule+0x3cb/0xed0 [12084.036616] schedule+0x4e/0xb0 [12084.036620] btrfs_start_ordered_extent+0x109/0x1c0 [btrfs] [12084.036651] ? prepare_to_wait_exclusive+0xc0/0xc0 [12084.036659] btrfs_run_ordered_extent_work+0x1a/0x30 [btrfs] [12084.036688] btrfs_work_helper+0xf8/0x400 [btrfs] [12084.0367 ---truncated---