bcachefs: fix seqmutex_relock()

We were grabbing the sequence number before unlock incremented it - fix
this by moving the increment to seqmutex_lock() (so the seqmutex_relock()
failure path skips the mutex_trylock()), and returning the sequence
number from unlock(), to make the API simpler and safer.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/debug.c

@@ -575,7 +575,6 @@ static ssize_t bch2_btree_transactions_read(struct file *file, char __user *buf,
 	struct bch_fs *c = i->c;
 	struct btree_trans *trans;
 	ssize_t ret = 0;
-	u32 seq;
 
 	i->ubuf = buf;
 	i->size	= size;
@@ -589,8 +588,7 @@ static ssize_t bch2_btree_transactions_read(struct file *file, char __user *buf,
 			continue;
 
 		closure_get(&trans->ref);
-		seq = seqmutex_seq(&c->btree_trans_lock);
-		seqmutex_unlock(&c->btree_trans_lock);
+		u32 seq = seqmutex_unlock(&c->btree_trans_lock);
 
 		ret = flush_buf(i);
 		if (ret) {
@@ -811,7 +809,6 @@ static ssize_t bch2_btree_deadlock_read(struct file *file, char __user *buf,
 	struct bch_fs *c = i->c;
 	struct btree_trans *trans;
 	ssize_t ret = 0;
-	u32 seq;
 
 	i->ubuf = buf;
 	i->size	= size;
@@ -828,8 +825,7 @@ static ssize_t bch2_btree_deadlock_read(struct file *file, char __user *buf,
 			continue;
 
 		closure_get(&trans->ref);
-		seq = seqmutex_seq(&c->btree_trans_lock);
-		seqmutex_unlock(&c->btree_trans_lock);
+		u32 seq = seqmutex_unlock(&c->btree_trans_lock);
 
 		ret = flush_buf(i);
 		if (ret) {

fs/bcachefs/seqmutex.h

@@ -19,17 +19,14 @@ static inline bool seqmutex_trylock(struct seqmutex *lock)
 static inline void seqmutex_lock(struct seqmutex *lock)
 {
 	mutex_lock(&lock->lock);
-}
-
-static inline void seqmutex_unlock(struct seqmutex *lock)
-{
 	lock->seq++;
-	mutex_unlock(&lock->lock);
 }
 
-static inline u32 seqmutex_seq(struct seqmutex *lock)
+static inline u32 seqmutex_unlock(struct seqmutex *lock)
 {
-	return lock->seq;
+	u32 seq = lock->seq;
+	mutex_unlock(&lock->lock);
+	return seq;
 }
 
 static inline bool seqmutex_relock(struct seqmutex *lock, u32 seq)