Make taskq_member() use ->journal_info

The ->journal_info pointer in the task_struct is reserved for use by
filesystems and because the kernel can have multiple file systems on the
same stack due to direct reclaim, each filesystem that touches
->journal_info in a callback function will save the value at the start
of its frame and restore it at the end of its frame.  This allows us to
safely use ->journal_info to store a pointer to the taskq's struct in
taskq threads so that ZFS code paths can detect the presence of a taskq.
This could break if the ZFS code were to use taskq_member from the
context of direct reclaim. However, there are no such uses of it in that
manner, so this is safe.

This eliminates an O(N) list traversal under a spinlock with an O(1)
unlocked pointer comparison.

Signed-off-by: Richard Yao <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Signed-off-by: tuxoko <[email protected]>
Signed-off-by: Tim Chase <[email protected]>
Closes openzfs#500

include/sys/taskq.h

@@ -124,7 +124,7 @@ extern void taskq_wait_id(taskq_t *, taskqid_t);
 extern void taskq_wait_outstanding(taskq_t *, taskqid_t);
 extern void taskq_wait(taskq_t *);
 extern int taskq_cancel_id(taskq_t *, taskqid_t);
-extern int taskq_member(taskq_t *, void *);
+#define	taskq_member(taskq, thread)	((taskq) == ((thread)->journal_info))
 
 #define taskq_create_proc(name, nthreads, pri, min, max, proc, flags) \
     taskq_create(name, nthreads, pri, min, max, flags)

module/spl/spl-taskq.c

@@ -448,40 +448,6 @@ taskq_wait(taskq_t *tq)
 }
 EXPORT_SYMBOL(taskq_wait);
 
-static int
-taskq_member_impl(taskq_t *tq, void *t)
-{
-	struct list_head *l;
-	taskq_thread_t *tqt;
-	int found = 0;
-
-	ASSERT(tq);
-	ASSERT(t);
-	ASSERT(spin_is_locked(&tq->tq_lock));
-
-	list_for_each(l, &tq->tq_thread_list) {
-		tqt = list_entry(l, taskq_thread_t, tqt_thread_list);
-		if (tqt->tqt_thread == (struct task_struct *)t) {
-			found = 1;
-			break;
-		}
-	}
-	return (found);
-}
-
-int
-taskq_member(taskq_t *tq, void *t)
-{
-	int found;
-
-	spin_lock_irqsave(&tq->tq_lock, tq->tq_lock_flags);
-	found = taskq_member_impl(tq, t);
-	spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
-
-	return (found);
-}
-EXPORT_SYMBOL(taskq_member);
-
 /*
  * Cancel an already dispatched task given the task id.  Still pending tasks
  * will be immediately canceled, and if the task is active the function will
@@ -812,6 +778,7 @@ taskq_thread(void *args)
 	ASSERT(tqt);
 	tq = tqt->tqt_tq;
 	current->flags |= PF_NOFREEZE;
+	current->journal_info = tq;
 
 	#if defined(PF_MEMALLOC_NOIO)
 	(void) memalloc_noio_save();
@@ -877,6 +844,8 @@ taskq_thread(void *args)
 			/* Perform the requested task */
 			t->tqent_func(t->tqent_arg);
 
+			ASSERT3P(tq, ==, current->journal_info);
+
 			spin_lock_irqsave(&tq->tq_lock, tq->tq_lock_flags);
 			tq->tq_nactive--;
 			list_del_init(&tqt->tqt_active_list);