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Date: Wed, 6 Jan 2010 21:42:25 +0100
From: Jan Kara <jack@...e.cz>
To: Theodore Ts'o <tytso@....edu>
Cc: Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: ext4/quota lockdep complaint: false positive?
Hi Ted,
On Sat 26-12-09 14:09:32, Theodore Ts'o wrote:
>
> While running the xfsqa regression test suite, lockdep triggered the
> following complaint, as shown below.
>
> I *think* it is a false positive, since the issue involves chown
> triggering a write to the quota file; and during the write, we are
> extending the quota file, which means grabbing i_data_sem while we have
> already grabbed the inode mutex for the quota file.
>
> The potential circular locking dependency that lockdep is complaining
> occurs only when we are mounting the filesystem, and reading from the
> quota file; and at that point we wouldn't be writing other files and
> needing to update quota file as aresult. So I don't think this is a
> problem in practice, but (1) i'd like a second opinion, and (2) I'm not
> sure what's the best way to make lockdep happy.
>
> Jan, what do you think?
The patch below should silence the lockdep warning.
Honza
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
---
>From 815e4d46ad7e56d41096116f5a6927903658551f Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@...e.cz>
Date: Wed, 6 Jan 2010 17:20:35 +0100
Subject: [PATCH] quota: Cleanup S_NOQUOTA handling
Cleanup handling of S_NOQUOTA inode flag and document it a bit. The flag
does not have to be set under dqptr_sem. Only functions modifying inode's
dquot pointers have to check the flag under dqptr_sem before going forward
with the modification. This way we are sure that we cannot add new dquot
pointers to the inode which is just becoming a quota file.
The good thing about this cleanup is that there are no more places in quota
code which enforce i_mutex vs. dqptr_sem lock ordering (in particular that
dqptr_sem -> i_mutex of quota file). This should silence some (false) lockdep
warnings with ext4 + quota and generally make life of some filesystems easier.
Signed-off-by: Jan Kara <jack@...e.cz>
---
fs/quota/dquot.c | 47 +++++++++++------------------------------------
1 files changed, 11 insertions(+), 36 deletions(-)
diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
index dea86ab..b4693f2 100644
--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -100,9 +100,13 @@
*
* Any operation working on dquots via inode pointers must hold dqptr_sem. If
* operation is just reading pointers from inode (or not using them at all) the
- * read lock is enough. If pointers are altered function must hold write lock
- * (these locking rules also apply for S_NOQUOTA flag in the inode - note that
- * for altering the flag i_mutex is also needed).
+ * read lock is enough. If pointers are altered function must hold write lock.
+ * Special care needs to be taken about S_NOQUOTA inode flag (marking that
+ * inode is a quota file). Functions adding pointers from inode to dquots have
+ * to check this flag under dqptr_sem and then (if S_NOQUOTA is not set) they
+ * have to do all pointer modifications before dropping dqptr_sem. This makes
+ * sure they cannot race with quotaon which first sets S_NOQUOTA flag and
+ * then drops all pointers to dquots from an inode.
*
* Each dquot has its dq_lock mutex. Locked dquots might not be referenced
* from inodes (dquot_alloc_space() and such don't check the dq_lock).
@@ -1275,7 +1279,6 @@ int dquot_initialize(struct inode *inode, int type)
}
down_write(&sb_dqopt(sb)->dqptr_sem);
- /* Having dqptr_sem we know NOQUOTA flags can't be altered... */
if (IS_NOQUOTA(inode))
goto out_err;
for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
@@ -1428,11 +1431,6 @@ int __dquot_alloc_space(struct inode *inode, qsize_t number,
}
down_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
- if (IS_NOQUOTA(inode)) {
- inode_incr_space(inode, number, reserve);
- goto out_unlock;
- }
-
for (cnt = 0; cnt < MAXQUOTAS; cnt++)
warntype[cnt] = QUOTA_NL_NOWARN;
@@ -1463,7 +1461,6 @@ int __dquot_alloc_space(struct inode *inode, qsize_t number,
mark_all_dquot_dirty(inode->i_dquot);
out_flush_warn:
flush_warnings(inode->i_dquot, warntype);
-out_unlock:
up_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
out:
return ret;
@@ -1496,10 +1493,6 @@ int dquot_alloc_inode(const struct inode *inode, qsize_t number)
for (cnt = 0; cnt < MAXQUOTAS; cnt++)
warntype[cnt] = QUOTA_NL_NOWARN;
down_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
- if (IS_NOQUOTA(inode)) {
- up_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
- return QUOTA_OK;
- }
spin_lock(&dq_data_lock);
for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
if (!inode->i_dquot[cnt])
@@ -1536,12 +1529,6 @@ int dquot_claim_space(struct inode *inode, qsize_t number)
}
down_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
- if (IS_NOQUOTA(inode)) {
- up_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
- inode_claim_rsv_space(inode, number);
- goto out;
- }
-
spin_lock(&dq_data_lock);
/* Claim reserved quotas to allocated quotas */
for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
@@ -1570,17 +1557,11 @@ int __dquot_free_space(struct inode *inode, qsize_t number, int reserve)
/* First test before acquiring mutex - solves deadlocks when we
* re-enter the quota code and are already holding the mutex */
if (IS_NOQUOTA(inode)) {
-out_sub:
inode_decr_space(inode, number, reserve);
return QUOTA_OK;
}
down_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
- /* Now recheck reliably when holding dqptr_sem */
- if (IS_NOQUOTA(inode)) {
- up_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
- goto out_sub;
- }
spin_lock(&dq_data_lock);
for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
if (!inode->i_dquot[cnt])
@@ -1633,11 +1614,6 @@ int dquot_free_inode(const struct inode *inode, qsize_t number)
return QUOTA_OK;
down_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
- /* Now recheck reliably when holding dqptr_sem */
- if (IS_NOQUOTA(inode)) {
- up_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
- return QUOTA_OK;
- }
spin_lock(&dq_data_lock);
for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
if (!inode->i_dquot[cnt])
@@ -1689,7 +1665,6 @@ int dquot_transfer(struct inode *inode, struct iattr *iattr)
GRPQUOTA);
down_write(&sb_dqopt(inode->i_sb)->dqptr_sem);
- /* Now recheck reliably when holding dqptr_sem */
if (IS_NOQUOTA(inode)) { /* File without quota accounting? */
up_write(&sb_dqopt(inode->i_sb)->dqptr_sem);
goto put_all;
@@ -2007,13 +1982,15 @@ static int vfs_load_quota_inode(struct inode *inode, int type, int format_id,
/* We don't want quota and atime on quota files (deadlocks
* possible) Also nobody should write to the file - we use
* special IO operations which ignore the immutable bit. */
- down_write(&dqopt->dqptr_sem);
mutex_lock_nested(&inode->i_mutex, I_MUTEX_QUOTA);
oldflags = inode->i_flags & (S_NOATIME | S_IMMUTABLE |
S_NOQUOTA);
inode->i_flags |= S_NOQUOTA | S_NOATIME | S_IMMUTABLE;
mutex_unlock(&inode->i_mutex);
- up_write(&dqopt->dqptr_sem);
+ /*
+ * When S_NOQUOTA is set, remove dquot references as no more
+ * references can be added
+ */
sb->dq_op->drop(inode);
}
@@ -2050,14 +2027,12 @@ out_file_init:
iput(inode);
out_lock:
if (oldflags != -1) {
- down_write(&dqopt->dqptr_sem);
mutex_lock_nested(&inode->i_mutex, I_MUTEX_QUOTA);
/* Set the flags back (in the case of accidental quotaon()
* on a wrong file we don't want to mess up the flags) */
inode->i_flags &= ~(S_NOATIME | S_NOQUOTA | S_IMMUTABLE);
inode->i_flags |= oldflags;
mutex_unlock(&inode->i_mutex);
- up_write(&dqopt->dqptr_sem);
}
mutex_unlock(&dqopt->dqonoff_mutex);
out_fmt:
--
1.6.4.2
--
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