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authorQu Wenruo <quwenruo@cn.fujitsu.com>2015-01-19 15:42:41 +0800
committerChris Mason <clm@fb.com>2015-01-20 17:20:21 -0800
commita53f4f8e9c8ebe6c9ee3b34c368913aae9876e22 (patch)
treedfe7b1c873db8cee0c10d3bbdd339b88f941ce88
parent6c9fe14f9d64cc12401a825a60ec5c5723496ca4 (diff)
downloadlinux-a53f4f8e9c8ebe6c9ee3b34c368913aae9876e22.tar.gz
btrfs: Don't call btrfs_start_transaction() on frozen fs to avoid deadlock.
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 <dsterba@suse.cz> Reported-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com> [enhanced comment, changed to SB_FREEZE_WRITE] Signed-off-by: David Sterba <dsterba@suse.cz> Signed-off-by: Chris Mason <clm@fb.com>
-rw-r--r--fs/btrfs/super.c10
1 files changed, 10 insertions, 0 deletions
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index c86fb543845..6f49b2872a6 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))