bcachefs: Fix race between trans_put() and btree_transactions_read()

debug.c was using closure_get() on a different thread's closure where
the we don't know if the object being refcounted is alive.

We keep btree_trans objects on a list so they can be printed by debug
code, and because it is cost prohibitive to touch the btree_trans list
every time we allocate and free btree_trans objects, cached objects are
also on this list.

However, we do not want the debug code to see cached but not in use
btree_trans objects - critically because the btree_paths array will have
been freed (if it was reallocated).

closure_get() is also incorrect to use when that get may race with it
hitting zero, i.e. we must already have a ref on the object or know the
ref can't currently hit 0 for other reasons (as used in the cycle
detector).

to fix this, use the previously introduced closure_get_not_zero(),
closure_return_sync(), and closure_init_stack_release(); the debug code
now can only take a ref on a trans object if it's alive and in use.

Signed-off-by: Kent Overstreet <[email protected]>

fs/bcachefs/btree_iter.c

@@ -3130,7 +3130,6 @@ struct btree_trans *__bch2_trans_get(struct bch_fs *c, unsigned fn_idx)
 
  trans = mempool_alloc(&c->btree_trans_pool, GFP_NOFS);
  memset(trans, 0, sizeof(*trans));
- closure_init_stack(&trans->ref);
 
  seqmutex_lock(&c->btree_trans_lock);
  if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG)) {
@@ -3161,7 +3160,6 @@ struct btree_trans *__bch2_trans_get(struct bch_fs *c, unsigned fn_idx)
 list_add_done:
  seqmutex_unlock(&c->btree_trans_lock);
 got_trans:
- trans->ref.closure_get_happened = false;
  trans->c = c;
  trans->last_begin_time = local_clock();
  trans->fn_idx = fn_idx;
@@ -3200,6 +3198,8 @@ struct btree_trans *__bch2_trans_get(struct bch_fs *c, unsigned fn_idx)
  trans->srcu_idx = srcu_read_lock(&c->btree_trans_barrier);
  trans->srcu_lock_time = jiffies;
  trans->srcu_held = true;
+
+ closure_init_stack_release(&trans->ref);
  return trans;
 }
 
@@ -3257,10 +3257,10 @@ void bch2_trans_put(struct btree_trans *trans)
  bch2_journal_keys_put(c);
 
  /*
- * trans->ref protects trans->locking_wait.task, btree_paths arary; used
+ * trans->ref protects trans->locking_wait.task, btree_paths array; used
  * by cycle detector
  */
- closure_sync(&trans->ref);
+ closure_return_sync(&trans->ref);
  trans->locking_wait.task = NULL;
 
  unsigned long *paths_allocated = trans->paths_allocated;
@@ -3385,8 +3385,6 @@ void bch2_fs_btree_iter_exit(struct bch_fs *c)
  per_cpu_ptr(c->btree_trans_bufs, cpu)->trans;
 
  if (trans) {
- closure_sync(&trans->ref);
-
  seqmutex_lock(&c->btree_trans_lock);
  list_del(&trans->list);
  seqmutex_unlock(&c->btree_trans_lock);

fs/bcachefs/debug.c

@@ -587,14 +587,10 @@ static ssize_t bch2_btree_transactions_read(struct file *file, char __user *buf,
  if (!task || task->pid <= i->iter)
  continue;
 
- closure_get(&trans->ref);
- u32 seq = seqmutex_unlock(&c->btree_trans_lock);
+ if (!closure_get_not_zero(&trans->ref))
+  continue;
 
- ret = flush_buf(i);
- if (ret) {
- closure_put(&trans->ref);
- goto unlocked;
- }
+ u32 seq = seqmutex_unlock(&c->btree_trans_lock);
 
  bch2_btree_trans_to_text(&i->buf, trans);
 
@@ -604,10 +600,12 @@ static ssize_t bch2_btree_transactions_read(struct file *file, char __user *buf,
  printbuf_indent_sub(&i->buf, 2);
  prt_newline(&i->buf);
 
- i->iter = task->pid;
-
  closure_put(&trans->ref);
 
+ ret = flush_buf(i);
+ if (ret)
+ goto unlocked;
+
  if (!seqmutex_relock(&c->btree_trans_lock, seq))
  goto restart;
  }
@@ -817,7 +815,8 @@ static void btree_deadlock_to_text(struct printbuf *out, struct bch_fs *c)
 
  iter = task->pid;
 
- closure_get(&trans->ref);
+ if (!closure_get_not_zero(&trans->ref))
+ continue;
 
  u32 seq = seqmutex_unlock(&c->btree_trans_lock);