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>

1 files changed, 48 insertions, 0 deletions

diff --git a/fs/bcachefs/seqmutex.h b/fs/bcachefs/seqmutex.h
new file mode 100644
index 000000000000..c1860d8163fb
--- /dev/null
+++ b/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 */