[PATCH] quota: fix a potential dead lock at add_dquot_ref() when performing quotaon against ext4 with files was writing

[Jeff Liu <jeff.liu@oracle.com>]

Hello,

I just ran into an issue on vanilla-kernel-3.3.0 when executing quotaon(8) on ext4 with "usrquota,grpquota", and there
have files are being writing at the same time.

I have a lxc guest running on an ext4 partition, 
$ mount|grep sda6
/dev/sda6 on /ext4 type ext4 (rw,usrquota,grpquota)

Execute quotacheck -cvumg /ext4 to force the quota checking without shutdown that guest, at this stage, everything is fine,
# quotacheck -cvgum /ext4
quotacheck: Your kernel probably supports journaled quota but you are not using it. Consider switching to journaled quota to avoid running quotacheck after an unclean shutdown.
quotacheck: Scanning /dev/sda6 [/ext4] done
quotacheck: Cannot stat old user quota file /ext4/aquota.user: No such file or directory. Usage will not be subtracted.
quotacheck: Cannot stat old group quota file /ext4/aquota.group: No such file or directory. Usage will not be subtracted.
quotacheck: Cannot stat old user quota file /ext4/aquota.user: No such file or directory. Usage will not be subtracted.
quotacheck: Cannot stat old group quota file /ext4/aquota.group: No such file or directory. Usage will not be subtracted.
quotacheck: Checked 3357 directories and 39335 files
quotacheck: Old file not found.
quotacheck: Old file not found.

However, the kernel was hang when running quotaon /ext4.

I observed the following info via netconsole:

[  423.140177]  *** DEADLOCK ***
[  423.140177] 
[  423.140177]  May be due to missing lock nesting notation
[  423.140177] 
[  423.140177] 4 locks held by quotaon/2350:
[  423.140177]  #0:  (&type->s_umount_key#26){++++++}, at: [<c122248e>] get_super+0xf9/0x1ec
[  423.140177]  #1:  (&s->s_dquot.dqonoff_mutex){+.+...}, at: [<c129b736>] vfs_load_quota_inode+0x3e5/0xac6
[  423.140177]  #2:  (inode_sb_list_lock){+.+...}, at: [<c129b99b>] vfs_load_quota_inode+0x64a/0xac6
[  423.140177]  #3:  (&sb->s_type->i_lock_key#16){+.+...}, at: [<c129b9de>] vfs_load_quota_inode+0x68d/0xac6
[  423.140177] 
[  423.140177] stack backtrace:
[  423.140177] Pid: 2350, comm: quotaon Not tainted 3.3.0 #79
[  423.140177] Call Trace:
[  423.140177]  [<c182b385>] ? printk+0x57/0x6a
[  423.140177]  [<c10ee73e>] __lock_acquire+0x133d/0x1a8a
[  423.140177]  [<c105da85>] ? vprintk+0x910/0x93a
[  423.140177]  [<c1298068>] ? inode_get_rsv_space+0x45/0x8b
[  423.140177]  [<c10ef795>] lock_acquire+0x13a/0x176
[  423.140177]  [<c1298068>] ? inode_get_rsv_space+0x45/0x8b
[  423.140177]  [<c182f778>] _raw_spin_lock+0x54/0x7d
[  423.140177]  [<c1298068>] ? inode_get_rsv_space+0x45/0x8b
[  423.140177]  [<c1298068>] inode_get_rsv_space+0x45/0x8b
[  423.140177]  [<c129bbe2>] vfs_load_quota_inode+0x891/0xac6
[  423.140177]  [<c129c1cb>] dquot_quota_on+0x82/0x97
[  423.140177]  [<f825d352>] ext4_quota_on+0x191/0x219 [ext4]
[  423.140177]  [<c106e4e5>] ? ns_capable+0x71/0xa3
[  423.140177]  [<c129e300>] do_quotactl+0x2f7/0x80f
[  423.140177]  [<f825d1c1>] ? ext4_msg+0x61/0x61 [ext4]
[  423.140177]  [<c122248e>] ? get_super+0xf9/0x1ec
[  423.140177]  [<c1222788>] ? get_super_thawed+0x33/0x147
[  423.140177]  [<c1246311>] ? iput+0x66/0x320
[  423.140177]  [<c129eaa8>] sys_quotactl+0x290/0x2fc
[  423.140177]  [<c18308ec>] syscall_call+0x7/0xb

As per my investigation, it occurred due to inode_get_rsv_space(inode) lock acquiring if the kernel was built with QUOTA_DEBUG enabled.
At add_dquot_ref(), the lock did not released if the inode.i_writecount is not *ZERO*, inode is not in I_FREEING|I_WILL_FREE|I_NEW state,
as well as it need to do quota init.

if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
     !atomic_read(&inode->i_writecount) ||
     !dqinit_needed(inode, type)) {
     spin_unlock(&inode->i_lock);
     continue;
}

#ifdef CONFIG_QUOTA_DEBUG
	if (unlikely(inode_get_rsv_space(inode) > 0))
        	reserved = 1;
#endif

In my situation, atomic_read(&inode->i_writecount) returns -2, looks that the related file have two
deny-writers via mmap at that time.

To nail down this issue, I refresh formated an ext4 file system, and try to open a file and keep writing to it(so that the atomic_read(&inode->i_writecount) = 1).
then perform quotacheck and quotaon at the same time, and repeat this process for 4 times, the kernel hang for 3 times, 1 time its ok.

# python -c "f=open('/ext4/test', 'w'); [(f.seek(x) or f.write(str(x))) for x in range(1, 1000000000, 99)]; f.close()"
# quotacheck -cvumg /ext4
quotacheck: Your kernel probably supports journaled quota but you are not using it. Consider switching to journaled quota to avoid running quotacheck after an unclean shutdown.
quotacheck: Scanning /dev/sda6 [/ext4] done
quotacheck: Cannot stat old user quota file /ext4/aquota.user: No such file or directory. Usage will not be subtracted.
quotacheck: Cannot stat old group quota file /ext4/aquota.group: No such file or directory. Usage will not be subtracted.
quotacheck: Cannot stat old user quota file /ext4/aquota.user: No such file or directory. Usage will not be subtracted.
quotacheck: Cannot stat old group quota file /ext4/aquota.group: No such file or directory. Usage will not be subtracted.
quotacheck: Checked 2 directories and 1 files
quotacheck: Old file not found.
quotacheck: Old file not found.
# quotaon /ext4			/* kernel was hang. */

I also verified that this issue can be reproduced against the latest kernel commit(Mon Apr 23 19:52:00 2012 95f714727436836bb46236ce2bcd8ee8f9274aed).

Below is a small patch, it looks a bit ugly, but works for me.

Thanks,
-Jeff

Signed-off-by: Jie Liu <jeff.liu@oracle.com>

---
 fs/quota/dquot.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
index 4674197..0ae7fc3 100644
--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -907,8 +907,10 @@ static void add_dquot_ref(struct super_block *sb, int type)
 			continue;
 		}
 #ifdef CONFIG_QUOTA_DEBUG
+		spin_unlock(&inode->i_lock);
 		if (unlikely(inode_get_rsv_space(inode) > 0))
 			reserved = 1;
+		spin_lock(&inode->i_lock);
 #endif
 		__iget(inode);
 		spin_unlock(&inode->i_lock);
-- 
1.7.9