[PATCH RT] fs/dcache: disable preemption on i_dir_seq's write side

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

i_dir_seq is an opencoded seqcounter. Based on the code it looks like we
could have two writers in parallel despite the fact that the d_lock is
held. The problem is that during the write process on RT the preemption
is still enabled and if this process is interrupted by a reader with RT
priority then we lock up.
To avoid that lock up I am disabling the preemption during the update.
The rename of i_dir_seq is here to ensure to catch new write sides in
future.

Cc: stable-rt@vger.kernel.org
Reported-by: Oleg.Karfich@wago.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 fs/dcache.c        |   12 +++++++-----
 fs/inode.c         |    2 +-
 fs/libfs.c         |    6 ++++--
 include/linux/fs.h |    2 +-
 4 files changed, 13 insertions(+), 9 deletions(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2405,9 +2405,10 @@ EXPORT_SYMBOL(d_rehash);
 static inline unsigned start_dir_add(struct inode *dir)
 {
 
+	preempt_disable_rt();
 	for (;;) {
-		unsigned n = dir->i_dir_seq;
-		if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
+		unsigned n = dir->__i_dir_seq;
+		if (!(n & 1) && cmpxchg(&dir->__i_dir_seq, n, n + 1) == n)
 			return n;
 		cpu_relax();
 	}
@@ -2415,7 +2416,8 @@ static inline unsigned start_dir_add(str
 
 static inline void end_dir_add(struct inode *dir, unsigned n)
 {
-	smp_store_release(&dir->i_dir_seq, n + 2);
+	smp_store_release(&dir->__i_dir_seq, n + 2);
+	preempt_enable_rt();
 }
 
 static void d_wait_lookup(struct dentry *dentry)
@@ -2448,7 +2450,7 @@ struct dentry *d_alloc_parallel(struct d
 
 retry:
 	rcu_read_lock();
-	seq = smp_load_acquire(&parent->d_inode->i_dir_seq) & ~1;
+	seq = smp_load_acquire(&parent->d_inode->__i_dir_seq) & ~1;
 	r_seq = read_seqbegin(&rename_lock);
 	dentry = __d_lookup_rcu(parent, name, &d_seq);
 	if (unlikely(dentry)) {
@@ -2470,7 +2472,7 @@ struct dentry *d_alloc_parallel(struct d
 		goto retry;
 	}
 	hlist_bl_lock(b);
-	if (unlikely(parent->d_inode->i_dir_seq != seq)) {
+	if (unlikely(parent->d_inode->__i_dir_seq != seq)) {
 		hlist_bl_unlock(b);
 		rcu_read_unlock();
 		goto retry;
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -154,7 +154,7 @@ int inode_init_always(struct super_block
 	inode->i_bdev = NULL;
 	inode->i_cdev = NULL;
 	inode->i_link = NULL;
-	inode->i_dir_seq = 0;
+	inode->__i_dir_seq = 0;
 	inode->i_rdev = 0;
 	inode->dirtied_when = 0;
 
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -90,7 +90,7 @@ static struct dentry *next_positive(stru
 				    struct list_head *from,
 				    int count)
 {
-	unsigned *seq = &parent->d_inode->i_dir_seq, n;
+	unsigned *seq = &parent->d_inode->__i_dir_seq, n;
 	struct dentry *res;
 	struct list_head *p;
 	bool skipped;
@@ -123,8 +123,9 @@ static struct dentry *next_positive(stru
 static void move_cursor(struct dentry *cursor, struct list_head *after)
 {
 	struct dentry *parent = cursor->d_parent;
-	unsigned n, *seq = &parent->d_inode->i_dir_seq;
+	unsigned n, *seq = &parent->d_inode->__i_dir_seq;
 	spin_lock(&parent->d_lock);
+	preempt_disable_rt();
 	for (;;) {
 		n = *seq;
 		if (!(n & 1) && cmpxchg(seq, n, n + 1) == n)
@@ -137,6 +138,7 @@ static void move_cursor(struct dentry *c
 	else
 		list_add_tail(&cursor->d_child, &parent->d_subdirs);
 	smp_store_release(seq, n + 2);
+	preempt_enable_rt();
 	spin_unlock(&parent->d_lock);
 }
 
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -651,7 +651,7 @@ struct inode {
 		struct block_device	*i_bdev;
 		struct cdev		*i_cdev;
 		char			*i_link;
-		unsigned		i_dir_seq;
+		unsigned		__i_dir_seq;
 	};
 
 	__u32			i_generation;