[PATCH v2] Btrfs: fix list_add corruption and soft lockups in fsync

[Liu Bo <bo.li.liu@oracle.com>]

Xfstests btrfs/146 revealed this corruption,

[   58.138831] Buffer I/O error on dev dm-0, logical block 2621424, async page read
[   58.151233] BTRFS error (device sdf): bdev /dev/mapper/error-test errs: wr 1, rd 0, flush 0, corrupt 0, gen 0
[   58.152403] list_add corruption. prev->next should be next (ffff88005e6775d8), but was ffffc9000189be88. (prev=ffffc9000189be88).
[   58.153518] ------------[ cut here ]------------
[   58.153892] WARNING: CPU: 1 PID: 1287 at lib/list_debug.c:31 __list_add_valid+0x169/0x1f0
...
[   58.157379] RIP: 0010:__list_add_valid+0x169/0x1f0
...
[   58.161956] Call Trace:
[   58.162264]  btrfs_log_inode_parent+0x5bd/0xfb0 [btrfs]
[   58.163583]  btrfs_log_dentry_safe+0x60/0x80 [btrfs]
[   58.164003]  btrfs_sync_file+0x4c2/0x6f0 [btrfs]
[   58.164393]  vfs_fsync_range+0x5f/0xd0
[   58.164898]  do_fsync+0x5a/0x90
[   58.165170]  SyS_fsync+0x10/0x20
[   58.165395]  entry_SYSCALL_64_fastpath+0x1f/0xbe
...

It turns out that we could record btrfs_log_ctx:io_err in
log_one_extents when IO fails, but make log_one_extents() return '0'
instead of -EIO, so the IO error is not acknowledged by the callers,
i.e.  btrfs_log_inode_parent(), which would remove btrfs_log_ctx:list
from list head 'root->log_ctxs'.  Since btrfs_log_ctx is allocated
from stack memory, it'd get freed with a object alive on the
list. then a future list_add will throw the above warning.

This returns the correct error in the above case.

Jeff also reported this while testing against his fsync error
patch set[1].

[1]: https://www.spinics.net/lists/linux-btrfs/msg65308.html
"btrfs list corruption and soft lockups while testing writeback error handling"

Fixes: 8407f553268a4611f254 ("Btrfs: fix data corruption after fast fsync and writeback error")
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
---
v2:
- Initialize ctx at the very beginning of fsync so that we can check
  if list is empty at out label

 fs/btrfs/file.c     | 5 +++--
 fs/btrfs/tree-log.c | 2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 504e96d..1e6956b 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2066,6 +2066,8 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 	len = (u64)end - (u64)start + 1;
 	trace_btrfs_sync_file(file, datasync);
 
+	btrfs_init_log_ctx(&ctx, inode);
+
 	/*
 	 * We write the dirty pages in the range and wait until they complete
 	 * out of the ->i_mutex. If so, we can flush the dirty pages by
@@ -2212,8 +2214,6 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 	}
 	trans->sync = true;
 
-	btrfs_init_log_ctx(&ctx, inode);
-
 	ret = btrfs_log_dentry_safe(trans, root, dentry, start, end, &ctx);
 	if (ret < 0) {
 		/* Fallthrough and commit/free transaction. */
@@ -2271,6 +2271,7 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 		ret = btrfs_end_transaction(trans);
 	}
 out:
+	ASSERT(list_empty(&ctx.list));
 	err = file_check_and_advance_wb_err(file);
 	if (!ret)
 		ret = err;
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index c800d06..d300284 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -4100,7 +4100,7 @@ static int log_one_extent(struct btrfs_trans_handle *trans,
 
 	if (ordered_io_err) {
 		ctx->io_err = -EIO;
-		return 0;
+		return ctx->io_err;
 	}
 
 	btrfs_init_map_token(&token);
-- 
2.9.4