linux-kernel - [PATCH v2] async: fix __lowest_in

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Date:	Tue, 22 Jan 2013 16:15:15 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ming Lei <ming.lei@...onical.com>,
	Alex Riesen <raa.lkml@...il.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Jens Axboe <axboe@...nel.dk>,
	USB list <linux-usb@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: [PATCH v2] async: fix __lowest_in_progress()

083b804c4d3e1e3d0eace56bdbc0f674946d2847 ("async: use workqueue for
worker pool") made it possible that async jobs are moved from pending
to running out-of-order.  While pending async jobs will be queued and
dispatched for execution in the same order, nothing guarantees they'll
enter "1) move self to the running queue" of async_run_entry_fn() in
the same order.

Before the conversion, async implemented its own worker pool.  An
async worker, upon being woken up, fetches the first item from the
pending list, which kept the executing lists sorted.  The conversion
to workqueue was done by adding work_struct to each async_entry and
async just schedules the work item.  The queueing and dispatching of
such work items are still in order but now each worker thread is
associated with a specific async_entry and moves that specific
async_entry to the executing list.  So, depending on which worker
reaches that point earlier, which is non-deterministic, we may end up
moving an async_entry with larger cookie before one with smaller one.

This broke __lowest_in_progress().  running->domain may not be
properly sorted and is not guaranteed to contain lower cookies than
pending list when not empty.  Fix it by ensuring sort-inserting to the
running list and always looking at both pending and running when
trying to determine the lowest cookie.

Over time, the async synchronization implementation became quite
messy.  We better restructure it such that each async_entry is linked
to two lists - one global and one per domain - and not move it when
execution starts.  There's no reason to distinguish pending and
running.  They behave the same for synchronization purposes.

v2: Description updated to better explain why it's broken.

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: stable@...r.kernel.org
---
Linus, I've updated the description to better explain why it's broken.
The code is ugly but cleanup patches are already ready, so it will be
cleaned up during 3.9-rc1.  How should this be routed?

Thanks.

 kernel/async.c |   27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

--- a/kernel/async.c
+++ b/kernel/async.c
@@ -86,18 +86,27 @@ static atomic_t entry_count;
  */
 static async_cookie_t  __lowest_in_progress(struct async_domain *running)
 {
+	async_cookie_t first_running = next_cookie;	/* infinity value */
+	async_cookie_t first_pending = next_cookie;	/* ditto */
 	struct async_entry *entry;

+	/*
+	 * Both running and pending lists are sorted but not disjoint.
+	 * Take the first cookies from both and return the min.
+	 */
 	if (!list_empty(&running->domain)) {
 		entry = list_first_entry(&running->domain, typeof(*entry), list);
-		return entry->cookie;
+		first_running = entry->cookie;
 	}

-	list_for_each_entry(entry, &async_pending, list)
-		if (entry->running == running)
-			return entry->cookie;
+	list_for_each_entry(entry, &async_pending, list) {
+		if (entry->running == running) {
+			first_pending = entry->cookie;
+			break;
+		}
+	}

-	return next_cookie;	/* "infinity" value */
+	return min(first_running, first_pending);
 }

 static async_cookie_t  lowest_in_progress(struct async_domain *running)
@@ -118,13 +127,17 @@ static void async_run_entry_fn(struct wo
 {
 	struct async_entry *entry =
 		container_of(work, struct async_entry, work);
+	struct async_entry *pos;
 	unsigned long flags;
 	ktime_t uninitialized_var(calltime), delta, rettime;
 	struct async_domain *running = entry->running;

-	/* 1) move self to the running queue */
+	/* 1) move self to the running queue, make sure it stays sorted */
 	spin_lock_irqsave(&async_lock, flags);
-	list_move_tail(&entry->list, &running->domain);
+	list_for_each_entry_reverse(pos, &running->domain, list)
+		if (entry->cookie < pos->cookie)
+			break;
+	list_move_tail(&entry->list, &pos->list);
 	spin_unlock_irqrestore(&async_lock, flags);

 	/* 2) run (and print duration) */
--
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