jffs2: fix lockdep warning

This patch series essentially fixes a false positive lockdep warning. The issue is known since 2012, referenced and fixed in the third patch. The other two patches subtly change the locking semantics on order to make the third patch feasible.

I would like thank Peter Zijlstra for his precise guiding on where to look. His first guess on what is going nailed it. I would also like to thank David Woodhouse and Richard Weinberger for their reviews.

Changes in v3:
- Avoid ?: operator (requested by Richard Weinberger).
- Add Reviewed-by tags from Richard Weinberger.
- Rebase patch series on v4.20-rc7.
- Update my email address.

Changes in v2:
- Initialize the lock class on every inode initialization rather than just the initial one in jffs2_i_init_once. (Thanks to David Woodhouse for pointing at this issue.)
- Properly wrap the commit message to make checkpatch.pl fully happy.

Helmut Grohne (3):
jffs2: avoid unnecessarily taking f->sem
jffs2: jffs2_iget defer f->sem acquisition
jffs2: put directories f->sem on a separate lockdep class

 fs/jffs2/fs.c        | 21 ++++++++++++++++++++-
 fs/jffs2/readinode.c |  4 +---
 2 files changed, 21 insertions(+), 4 deletions(-)

2.11.0

jffs2_xattr_do_crccheck_inode creates a temporary struct jffs2_inode_info for handling it to jffs2_do_read_inode_internal. Formerly, jffs2_do_read_inode_internal released f->sem in the error path and that was why it had to be initialized, taken and released. As part of 7aaea7605c0 ("jffs2: fix unbalanced locking") locking was moved out of it.

In the relevant jffs2_xattr_do_crccheck_inode we own the only reference to f as we allocate it with kmalloc. Since nothing else has knowledge of it, there is no need to lock it. In particular, that saves us from thinking about the correct locking class.

We still need to initialize the mutex as jffs2_do_clear_inode insists on acquiring it.

---
 fs/jffs2/readinode.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

index 389ea53ea487..a6718fbfe1fd 100644
@@ -1382,12 +1382,10 @@ int jffs2_do_crccheck_inode(struct jffs2_sb_info *c, struct jffs2_inode_cache *i
 	if (!f)
 		return -ENOMEM;
 
-	mutex_init(&f->sem);
-	mutex_lock(&f->sem);
 	f->inocache = ic;
 
 	ret = jffs2_do_read_inode_internal(c, f, &n);
-	mutex_unlock(&f->sem);
+	mutex_init(&f->sem);
 	jffs2_do_clear_inode(c, f);
 	jffs2_xattr_do_crccheck_inode(c, ic);
 	kfree (f);

2.11.0

At the point where f->sem is taken, we know that we have a locked I_NEW inode. Since jffs2_iget is the only function processing I_NEW inodes and iget_locked blocks on I_NEW inodes, we know that we are the only user of the inode until we unlock_new_inode it. Since 7aaea7605c0 ("jffs2: fix unbalanced locking"), jffs2_do_read_inode no longer touches f->sem and does not make the inode visible to others either. Thus we can call it without f->sem acquired. Nextup, inode->i_mode is protected until unlock_new_inode.

After deferring the locking, we can decide upon the proper lock class depending on inode->i_mode.

---
 fs/jffs2/fs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

index eab04eca95a3..89a10b398d00 100644
@@ -270,13 +270,15 @@ struct inode *jffs2_iget(struct super_block *sb, unsigned long ino)
 	c = JFFS2_SB_INFO(inode->i_sb);
 
 	jffs2_init_inode_info(f);
-	mutex_lock(&f->sem);
 
 	ret = jffs2_do_read_inode(c, f, inode->i_ino, &latest_node);
 	if (ret)
 		goto error;
 
 	inode->i_mode = jemode_to_cpu(latest_node.mode);
+
+	mutex_lock(&f->sem);
+
 	i_uid_write(inode, je16_to_cpu(latest_node.uid));
 	i_gid_write(inode, je16_to_cpu(latest_node.gid));
 	inode->i_size = je32_to_cpu(latest_node.isize);

2.11.0

We get a lockdep warning if we read a directory (e.g. ls) and fault a page on a regular file:

[  181.206207] mmaptest/392 is trying to acquire lock:
[  181.211066]  (&f->sem){+.+.}, at: [<c01ff7d8>] jffs2_readpage+0x30/0x5c
[  181.217663]
[  181.217663] but task is already holding lock:
[  181.223477]  (&mm->mmap_sem){++++}, at: [<c04928e8>] do_page_fault+0xa8/0x454
[  181.230596]
[  181.230596] which lock already depends on the new lock.
[  181.230596]
[  181.238755]
[  181.238755] the existing dependency chain (in reverse order) is:
[  181.246219]
[  181.246219] -> #1 (&mm->mmap_sem){++++}:
[  181.251614]        __might_fault+0x90/0xc0
[  181.255694]        filldir+0xa4/0x1f4
[  181.259334]        jffs2_readdir+0xd8/0x1d4
[  181.263499]        iterate_dir+0x78/0x128
[  181.267490]        SyS_getdents+0x8c/0x12c
[  181.271576]        ret_fast_syscall+0x0/0x28
[  181.275818]
[  181.275818] -> #0 (&f->sem){+.+.}:
[  181.280690]        lock_acquire+0xfc/0x2b0
[  181.284763]        __mutex_lock+0x8c/0xb0c
[  181.288842]        mutex_lock_nested+0x2c/0x34
[  181.293272]        jffs2_readpage+0x30/0x5c
[  181.297438]        filemap_fault+0x600/0x73c
[  181.301690]        __do_fault+0x28/0xb8
[  181.305510]        handle_mm_fault+0x854/0xec0
[  181.309937]        do_page_fault+0x384/0x454
[  181.314188]        do_DataAbort+0x4c/0xc8
[  181.318181]        __dabt_usr+0x44/0x60
[  181.321999]        0xb6dffb02

Like with inode->i_rwsem, we recognize that this can never result in a dead lock, because we cannot page fault directory inodes and we cannot call getdents on non-directories. Thus we need different lockdep classes depending on the mode. This mirrors what lockdep_annotate_inode_mutex_key does to inodes.

---
 fs/jffs2/fs.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

index 89a10b398d00..d4d691488a19 100644
@@ -26,6 +26,9 @@
 #include <linux/crc32.h>
 #include "nodelist.h"
 
+static struct lock_class_key jffs2_inode_info_sem_key,
+	 jffs2_inode_info_sem_dir_key;
+
 static int jffs2_flash_setup(struct jffs2_sb_info *c);
 
 int jffs2_do_setattr (struct inode *inode, struct iattr *iattr)

@@ -277,6 +280,13 @@ struct inode *jffs2_iget(struct super_block *sb, unsigned long ino)
 
 	inode->i_mode = jemode_to_cpu(latest_node.mode);
 
+	/* Mirror lock class of inode->i_rwsem, see
+	 * lockdep_annotate_inode_mutex_key.
+	 */
+	if (S_ISDIR(inode->i_mode))
+		lockdep_set_class(&f->sem, &jffs2_inode_info_sem_dir_key);
+	else
+		lockdep_set_class(&f->sem, &jffs2_inode_info_sem_key);
 	mutex_lock(&f->sem);
 
 	i_uid_write(inode, je16_to_cpu(latest_node.uid));

@@ -439,6 +449,13 @@ struct inode *jffs2_new_inode (struct inode *dir_i, umode_t mode, struct jffs2_r
 
 	f = JFFS2_INODE_INFO(inode);
 	jffs2_init_inode_info(f);
+	/* Mirror lock class of inode->i_rwsem, see
+	 * lockdep_annotate_inode_mutex_key.
+	 */
+	if (S_ISDIR(inode->i_mode))
+		lockdep_set_class(&f->sem, &jffs2_inode_info_sem_dir_key);
+	else
+		lockdep_set_class(&f->sem, &jffs2_inode_info_sem_key);
 	mutex_lock(&f->sem);
 
 	memset(ri, 0, sizeof(*ri));

2.11.0