bcachefs: Take a SRCU lock in btree transactions

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

fs/bcachefs/Kconfig

@@ -19,6 +19,7 @@ config BCACHEFS_FS
 	select KEYS
 	select RAID6_PQ
 	select XOR_BLOCKS
+	select SRCU
 	help
 	The bcachefs filesystem - a modern, copy on write filesystem, with
 	support for multiple devices, compression, checksumming, etc.

fs/bcachefs/bcachefs.h

@@ -193,6 +193,7 @@
 #include <linux/rwsem.h>
 #include <linux/seqlock.h>
 #include <linux/shrinker.h>
+#include <linux/srcu.h>
 #include <linux/types.h>
 #include <linux/workqueue.h>
 #include <linux/zstd.h>
@@ -642,6 +643,8 @@ struct bch_fs {
 	mempool_t		btree_iters_pool;
 	struct btree_iter_buf  __percpu	*btree_iters_bufs;
 
+	struct srcu_struct	btree_trans_barrier;
+
 	struct btree_key_cache	btree_key_cache;
 
 	struct workqueue_struct	*wq;

fs/bcachefs/btree_iter.c

@@ -2373,6 +2373,8 @@ void bch2_trans_init(struct btree_trans *trans, struct bch_fs *c,
 	if (expected_mem_bytes)
 		bch2_trans_preload_mem(trans, expected_mem_bytes);
 
+	trans->srcu_idx = srcu_read_lock(&c->btree_trans_barrier);
+
 #ifdef CONFIG_BCACHEFS_DEBUG
 	trans->pid = current->pid;
 	mutex_lock(&c->btree_trans_lock);
@@ -2393,6 +2395,8 @@ int bch2_trans_exit(struct btree_trans *trans)
 	mutex_unlock(&trans->c->btree_trans_lock);
 #endif
 
+	srcu_read_unlock(&c->btree_trans_barrier, trans->srcu_idx);
+
 	bch2_journal_preres_put(&trans->c->journal, &trans->journal_preres);
 
 	kfree(trans->fs_usage_deltas);
@@ -2475,6 +2479,7 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct bch_fs *c)
 void bch2_fs_btree_iter_exit(struct bch_fs *c)
 {
 	mempool_exit(&c->btree_iters_pool);
+	cleanup_srcu_struct(&c->btree_trans_barrier);
 }
 
 int bch2_fs_btree_iter_init(struct bch_fs *c)
@@ -2484,7 +2489,8 @@ int bch2_fs_btree_iter_init(struct bch_fs *c)
 	INIT_LIST_HEAD(&c->btree_trans_list);
 	mutex_init(&c->btree_trans_lock);
 
-	return mempool_init_kmalloc_pool(&c->btree_iters_pool, 1,
+	return  init_srcu_struct(&c->btree_trans_barrier) ?:
+		mempool_init_kmalloc_pool(&c->btree_iters_pool, 1,
 			sizeof(struct btree_iter) * nr +
 			sizeof(struct btree_insert_entry) * nr +
 			sizeof(struct btree_insert_entry) * nr);

fs/bcachefs/btree_types.h

@@ -350,6 +350,7 @@ struct btree_trans {
 	pid_t			pid;
 #endif
 	unsigned long		ip;
+	int			srcu_idx;
 
 	u64			iters_linked;
 	u64			iters_live;