bcachefs: Inject transaction restarts in debug mode

In CONFIG_BCACHEFS_DEBUG mode, we'll now randomly issue transaction
restarts - with a decaying probability based on the number of restarts
we've already had, to ensure that transactions eventually make forward
progress. This should help shake out some bugs.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -17,6 +17,7 @@
 #include "subvolume.h"
 #include "trace.h"
 
+#include <linux/random.h>
 #include <linux/prefetch.h>
 
 static inline void btree_path_list_remove(struct btree_trans *, struct btree_path *);
@@ -1660,6 +1661,16 @@ static int btree_path_traverse_one(struct btree_trans *trans,
 int __must_check bch2_btree_path_traverse(struct btree_trans *trans,
 					  struct btree_path *path, unsigned flags)
 {
+	if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG)) {
+		unsigned restart_probability_bits = 4 << min(trans->restart_count, 32U);
+		u64 max = ~(~0ULL << restart_probability_bits);
+
+		if (!get_random_u32_below(max)) {
+			trace_transaction_restart_injected(trans->fn, _RET_IP_);
+			return btree_trans_restart(trans, BCH_ERR_transaction_restart_fault_inject);
+		}
+	}
+
 	if (path->uptodate < BTREE_ITER_NEED_RELOCK)
 		return 0;
 

fs/bcachefs/trace.h

@@ -707,6 +707,12 @@ DEFINE_EVENT(transaction_event,	transaction_restart_ip,
 	TP_ARGS(trans_fn, caller_ip)
 );
 
+DEFINE_EVENT(transaction_event,	transaction_restart_injected,
+	TP_PROTO(const char *trans_fn,
+		 unsigned long caller_ip),
+	TP_ARGS(trans_fn, caller_ip)
+);
+
 DEFINE_EVENT(transaction_event,	trans_blocked_journal_reclaim,
 	TP_PROTO(const char *trans_fn,
 		 unsigned long caller_ip),