bcachefs: seqmutex; fix a lockdep splat

We can't be holding btree_trans_lock while copying to user space, which
might incur a page fault. To fix this, convert it to a seqmutex so we
can unlock/relock.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -208,6 +208,7 @@
 #include "fifo.h"
 #include "nocow_locking_types.h"
 #include "opts.h"
+#include "seqmutex.h"
 #include "util.h"
 
 #ifdef CONFIG_BCACHEFS_DEBUG
@@ -779,7 +780,7 @@ struct bch_fs {
 	}			btree_write_stats[BTREE_WRITE_TYPE_NR];
 
 	/* btree_iter.c: */
-	struct mutex		btree_trans_lock;
+	struct seqmutex		btree_trans_lock;
 	struct list_head	btree_trans_list;
 	mempool_t		btree_paths_pool;
 	mempool_t		btree_trans_mem_pool;

fs/bcachefs/btree_iter.c

@@ -2991,7 +2991,7 @@ void __bch2_trans_init(struct btree_trans *trans, struct bch_fs *c, unsigned fn_
 	if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG_TRANSACTIONS)) {
 		struct btree_trans *pos;
 
-		mutex_lock(&c->btree_trans_lock);
+		seqmutex_lock(&c->btree_trans_lock);
 		list_for_each_entry(pos, &c->btree_trans_list, list) {
 			/*
 			 * We'd much prefer to be stricter here and completely
@@ -3009,7 +3009,7 @@ void __bch2_trans_init(struct btree_trans *trans, struct bch_fs *c, unsigned fn_
 		}
 		list_add_tail(&trans->list, &c->btree_trans_list);
 list_add_done:
-		mutex_unlock(&c->btree_trans_lock);
+		seqmutex_unlock(&c->btree_trans_lock);
 	}
 }
 
@@ -3044,6 +3044,12 @@ void bch2_trans_exit(struct btree_trans *trans)
 
 	bch2_trans_unlock(trans);
 
+	if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG_TRANSACTIONS)) {
+		seqmutex_lock(&c->btree_trans_lock);
+		list_del(&trans->list);
+		seqmutex_unlock(&c->btree_trans_lock);
+	}
+
 	closure_sync(&trans->ref);
 
 	if (s)
@@ -3055,12 +3061,6 @@ void bch2_trans_exit(struct btree_trans *trans)
 
 	check_btree_paths_leaked(trans);
 
-	if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG_TRANSACTIONS)) {
-		mutex_lock(&c->btree_trans_lock);
-		list_del(&trans->list);
-		mutex_unlock(&c->btree_trans_lock);
-	}
-
 	srcu_read_unlock(&c->btree_trans_barrier, trans->srcu_idx);
 
 	bch2_journal_preres_put(&c->journal, &trans->journal_preres);
@@ -3198,7 +3198,7 @@ int bch2_fs_btree_iter_init(struct bch_fs *c)
 	}
 
 	INIT_LIST_HEAD(&c->btree_trans_list);
-	mutex_init(&c->btree_trans_lock);
+	seqmutex_init(&c->btree_trans_lock);
 
 	ret   = mempool_init_kmalloc_pool(&c->btree_paths_pool, 1,
 			sizeof(struct btree_path) * nr +

fs/bcachefs/debug.c

@@ -627,19 +627,26 @@ 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;
 	i->ret	= 0;
-
-	mutex_lock(&c->btree_trans_lock);
+restart:
+	seqmutex_lock(&c->btree_trans_lock);
 	list_for_each_entry(trans, &c->btree_trans_list, list) {
 		if (trans->locking_wait.task->pid <= i->iter)
 			continue;
 
+		closure_get(&trans->ref);
+		seq = seqmutex_seq(&c->btree_trans_lock);
+		seqmutex_unlock(&c->btree_trans_lock);
+
 		ret = flush_buf(i);
-		if (ret)
-			break;
+		if (ret) {
+			closure_put(&trans->ref);
+			goto unlocked;
+		}
 
 		bch2_btree_trans_to_text(&i->buf, trans);
 
@@ -651,9 +658,14 @@ static ssize_t bch2_btree_transactions_read(struct file *file, char __user *buf,
 		prt_newline(&i->buf);
 
 		i->iter = trans->locking_wait.task->pid;
-	}
-	mutex_unlock(&c->btree_trans_lock);
 
+		closure_put(&trans->ref);
+
+		if (!seqmutex_relock(&c->btree_trans_lock, seq))
+			goto restart;
+	}
+	seqmutex_unlock(&c->btree_trans_lock);
+unlocked:
 	if (i->buf.allocation_failure)
 		ret = -ENOMEM;
 
@@ -815,6 +827,7 @@ 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;
@@ -822,21 +835,32 @@ static ssize_t bch2_btree_deadlock_read(struct file *file, char __user *buf,
 
 	if (i->iter)
 		goto out;
-
-	mutex_lock(&c->btree_trans_lock);
+restart:
+	seqmutex_lock(&c->btree_trans_lock);
 	list_for_each_entry(trans, &c->btree_trans_list, list) {
 		if (trans->locking_wait.task->pid <= i->iter)
 			continue;
 
+		closure_get(&trans->ref);
+		seq = seqmutex_seq(&c->btree_trans_lock);
+		seqmutex_unlock(&c->btree_trans_lock);
+
 		ret = flush_buf(i);
-		if (ret)
-			break;
+		if (ret) {
+			closure_put(&trans->ref);
+			goto out;
+		}
 
 		bch2_check_for_deadlock(trans, &i->buf);
 
 		i->iter = trans->locking_wait.task->pid;
+
+		closure_put(&trans->ref);
+
+		if (!seqmutex_relock(&c->btree_trans_lock, seq))
+			goto restart;
 	}
-	mutex_unlock(&c->btree_trans_lock);
+	seqmutex_unlock(&c->btree_trans_lock);
 out:
 	if (i->buf.allocation_failure)
 		ret = -ENOMEM;

fs/bcachefs/seqmutex.h

@@ -0,0 +1,48 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _BCACHEFS_SEQMUTEX_H
+#define _BCACHEFS_SEQMUTEX_H
+
+#include <linux/mutex.h>
+
+struct seqmutex {
+	struct mutex	lock;
+	u32		seq;
+};
+
+#define seqmutex_init(_lock)	mutex_init(&(_lock)->lock)
+
+static inline bool seqmutex_trylock(struct seqmutex *lock)
+{
+	return mutex_trylock(&lock->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)
+{
+	return lock->seq;
+}
+
+static inline bool seqmutex_relock(struct seqmutex *lock, u32 seq)
+{
+	if (lock->seq != seq || !mutex_trylock(&lock->lock))
+		return false;
+
+	if (lock->seq != seq) {
+		mutex_unlock(&lock->lock);
+		return false;
+	}
+
+	return true;
+}
+
+#endif /* _BCACHEFS_SEQMUTEX_H */

fs/bcachefs/sysfs.c

@@ -379,7 +379,7 @@ static void bch2_btree_wakeup_all(struct bch_fs *c)
 {
 	struct btree_trans *trans;
 
-	mutex_lock(&c->btree_trans_lock);
+	seqmutex_lock(&c->btree_trans_lock);
 	list_for_each_entry(trans, &c->btree_trans_list, list) {
 		struct btree_bkey_cached_common *b = READ_ONCE(trans->locking);
 
@@ -387,7 +387,7 @@ static void bch2_btree_wakeup_all(struct bch_fs *c)
 			six_lock_wakeup_all(&b->lock);
 
 	}
-	mutex_unlock(&c->btree_trans_lock);
+	seqmutex_unlock(&c->btree_trans_lock);
 }
 
 SHOW(bch2_fs)