From: Qu Wenruo <quwen...@cn.fujitsu.com>
Commit 6b5fe46dfa52 (btrfs: do commit in sync_fs if there are pending
changes) will call btrfs_start_transaction() in sync_fs(), to handle
some operations needed to be done in next transaction.
However this can cause deadlock if the filesystem is frozen, with the
following sys_r+w output:
[ 143.255932] Call Trace:
[ 143.255936] [<ffffffff816c0e09>] schedule+0x29/0x70
[ 143.255939] [<ffffffff811cb7f3>] __sb_start_write+0xb3/0x100
[ 143.255971] [<ffffffffa040ec06>] start_transaction+0x2e6/0x5a0
[btrfs]
[ 143.255992] [<ffffffffa040f1eb>] btrfs_start_transaction+0x1b/0x20
[btrfs]
[ 143.256003] [<ffffffffa03dc0ba>] btrfs_sync_fs+0xca/0xd0 [btrfs]
[ 143.256007] [<ffffffff811f7be0>] sync_fs_one_sb+0x20/0x30
[ 143.256011] [<ffffffff811cbd01>] iterate_supers+0xe1/0xf0
[ 143.256014] [<ffffffff811f7d75>] sys_sync+0x55/0x90
[ 143.256017] [<ffffffff816c49d2>] system_call_fastpath+0x12/0x17
[ 143.256111] Call Trace:
[ 143.256114] [<ffffffff816c0e09>] schedule+0x29/0x70
[ 143.256119] [<ffffffff816c3405>] rwsem_down_write_failed+0x1c5/0x2d0
[ 143.256123] [<ffffffff8133f013>] call_rwsem_down_write_failed+0x13/0x20
[ 143.256131] [<ffffffff811caae8>] thaw_super+0x28/0xc0
[ 143.256135] [<ffffffff811db3e5>] do_vfs_ioctl+0x3f5/0x540
[ 143.256187] [<ffffffff811db5c1>] SyS_ioctl+0x91/0xb0
[ 143.256213] [<ffffffff816c49d2>] system_call_fastpath+0x12/0x17
The reason is like the following:
(Holding s_umount)
VFS sync_fs staff:
|- btrfs_sync_fs()
|- btrfs_start_transaction()
|- sb_start_intwrite()
(Waiting thaw_fs to unfreeze)
VFS thaw_fs staff:
thaw_fs()
(Waiting sync_fs to release
s_umount)
So deadlock happens.
This can be easily triggered by fstest/generic/068 with inode_cache
mount option.
The fix is to check if the fs is frozen, if the fs is frozen, just
return and waiting for the next transaction.
Cc: David Sterba <dste...@suse.cz>
Reported-by: Gui Hecheng <guihc.f...@cn.fujitsu.com>
Signed-off-by: Qu Wenruo <quwen...@cn.fujitsu.com>
[enhanced comment, changed to SB_FREEZE_WRITE]
Signed-off-by: David Sterba <dste...@suse.cz>
---
fs/btrfs/super.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index c86fb5438454..6f49b2872a64 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1000,6 +1000,16 @@ int btrfs_sync_fs(struct super_block *sb, int wait)
*/
if (fs_info->pending_changes == 0)
return 0;
+ /*
+ * A non-blocking test if the fs is frozen. We must not
+ * start a new transaction here otherwise a deadlock
+ * happens. The pending operations are delayed to the
+ * next commit after thawing.
+ */
+ if (__sb_start_write(sb, SB_FREEZE_WRITE, false))
+ __sb_end_write(sb, SB_FREEZE_WRITE);
+ else
+ return 0;
trans = btrfs_start_transaction(root, 0);
}
if (IS_ERR(trans))
--
2.1.3
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