linux-kernel - Re: [PATCH] sched: Avoid that __wait_on_bit

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Date:	Fri, 12 Aug 2016 18:16:12 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Bart Van Assche <bart.vanassche@...disk.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Neil Brown <neilb@...e.de>,
	Michael Shaver <jmshaver@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs

On 08/11, Oleg Nesterov wrote:
>
> I'll send another debugging patch tomorrow, I was a bit busy today. The next
> step is obvious, we need to know the caller.

Please drop two patches I sent before anf try the new one below.

Which kernel version do you use?

Oleg.
---

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index e5a3244..533da3ab 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -711,6 +711,15 @@ static inline int page_has_private(struct page *page)
 	return !!(page->flags & PAGE_FLAGS_PRIVATE);
 }
 
+void unlock_page_x(struct page *page);
+static inline void __ClearPageLocked_x(struct page *page)
+{
+	if (PageLocked(compound_head(page)))
+		unlock_page_x(page);
+}
+
+#define __ClearPageLocked(page)	__ClearPageLocked_x(page)
+
 #undef PF_ANY
 #undef PF_HEAD
 #undef PF_NO_TAIL
diff --git a/mm/filemap.c b/mm/filemap.c
index 20f3b1f..fb320fb 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -837,6 +837,43 @@ void unlock_page(struct page *page)
 }
 EXPORT_SYMBOL(unlock_page);
 
+void unlock_page_x(struct page *__page)
+{
+	struct page *page = compound_head(__page);
+	wait_queue_head_t *wq = page_waitqueue(page);
+	struct wait_bit_key key = __WAIT_BIT_KEY_INITIALIZER(&page->flags, PG_locked);
+	wait_queue_t *curr, *next;
+	unsigned long flags;
+	bool w = false;
+
+	#define W() do {								\
+		if (!w) { w = true; pr_crit("XXXXXXXXXXXX\n"); dump_stack(); }	\
+	} while (0)
+
+	clear_bit_unlock(PG_locked, &page->flags);
+	smp_mb__after_atomic();
+
+	if (!waitqueue_active(wq))
+		return;
+
+	spin_lock_irqsave(&wq->lock, flags);
+	list_for_each_entry_safe(curr, next, &wq->task_list, task_list) {
+		if (curr->func == wake_bit_function) {
+			struct wait_bit_queue *wb = container_of(curr, struct wait_bit_queue, wait);
+			if (wb->key.flags == key.flags && wb->key.bit_nr == PG_locked) {
+				W();
+				pr_crit("XXX flags = %x, waiter:\n", curr->flags);
+				sched_show_task(curr->private);
+			}
+		} else {
+			W();
+			pr_crit("XXX flags = %x, func = %pF\n", curr->flags, curr->func);
+		}
+		curr->func(curr, TASK_NORMAL, 0, &key);
+	}
+	spin_unlock_irqrestore(&wq->lock, flags);
+}
+
 /**
  * end_page_writeback - end writeback against a page
  * @page: the page