Hello,
I am looking at the fs quota code while debugging a deadlock problem
on ext2 file system. I found there is a potential deadlock between quotaon
and quotaoff (or quotasync). Basically, all of quotactl operations need to
be protected by dqonoff_mutex. vfs_quota_off and vfs_quota_sync also call
sb->s_op->quota_write that needs to grab the i_mutex of the quota file.
But in vfs_quota_on_inode (called from quotaon operation), the current
code tries to grab the i_mutex of the quota file first before getting
quonoff_mutex.
Here is a simple test to show the problem:
$ while true; do quotaon /dev/hda >&/dev/null; usleep $RANDOM; done &
$ while true; do quotaoff /dev/hda >&/dev/null; usleep $RANDOM; done &
After running for a while, the two processes get deadlocked.
Below is my proposed change to fix the problem:
Index: git-linux/fs/quota/dquot.c
===================================================================
--- git-linux.orig/fs/quota/dquot.c 2009-05-20 18:05:55.000000000 -0700
+++ git-linux/fs/quota/dquot.c 2009-06-26 17:57:04.000000000 -0700
@@ -2042,8 +2042,8 @@
* changes */
invalidate_bdev(sb->s_bdev);
}
- mutex_lock(&inode->i_mutex);
mutex_lock(&dqopt->dqonoff_mutex);
+ mutex_lock(&inode->i_mutex);
if (sb_has_quota_loaded(sb, type)) {
error = -EBUSY;
goto out_lock;
@@ -2094,7 +2094,6 @@
dqopt->files[type] = NULL;
iput(inode);
out_lock:
- mutex_unlock(&dqopt->dqonoff_mutex);
if (oldflags != -1) {
down_write(&dqopt->dqptr_sem);
/* Set the flags back (in the case of accidental quotaon()
@@ -2104,6 +2103,7 @@
up_write(&dqopt->dqptr_sem);
}
mutex_unlock(&inode->i_mutex);
+ mutex_unlock(&dqopt->dqonoff_mutex);
out_fmt:
put_quota_format(fmt);
Also while debugging the problem, I found the following code path:
shrink_icache_memory -> prune_icache (grab iprune_mutex) -> dispose_list ->
clear_inode -> dquot_drop -> dqput -> dquot_release ->
dqopt->ops[dquot->dq_type]->write_file_info ->
sb->s_op->quota_write (i.e., ext2_quota_write for ext2)
AFAICT, it seems very deadlock prone that the quota system tries to write
the quota file while clearing an inode. The ext2_quota_write calls
ext2_get_block that may block at alloc_page that in turn may try to call
shrink_icache_memory when the system is low in memory. But the
iprune_mutex is already hold by the process so the system will get
into deadlock here. I haven't got a test case that shows the deadlock but
want to raise this issue here first to see if I have missed anything.
Jiaying
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