linux-kernel - [ 032/184] Fix a dead loop in async_synchronize

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Message-Id: <20130604172131.637553911@1wt.eu>
Date:	Tue, 04 Jun 2013 19:22:02 +0200
From:	Willy Tarreau <w@....eu>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...il.com>,
	Christian Kujau <lists@...dbynature.de>,
	Cong Wang <xiyou.wangcong@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Willy Tarreau <w@....eu>
Subject: [ 032/184] Fix a dead loop in async_synchronize_full()

2.6.32-longterm review patch.  If anyone has any objections, please let me know.

------------------

From: Li Zhong <zhong@...ux.vnet.ibm.com>

[Fixed upstream by commits 2955b47d2c1983998a8c5915cb96884e67f7cb53 and
a4683487f90bfe3049686fc5c566bdc1ad03ace6 from Dan Williams, but they are much
more intrusive than this tiny fix, according to Andrew - gregkh]

This patch tries to fix a dead loop in  async_synchronize_full(), which
could be seen when preemption is disabled on a single cpu machine.

void async_synchronize_full(void)
{
        do {
                async_synchronize_cookie(next_cookie);
        } while (!list_empty(&async_running) || !
list_empty(&async_pending));
}

async_synchronize_cookie() calls async_synchronize_cookie_domain() with
&async_running as the default domain to synchronize.

However, there might be some works in the async_pending list from other
domains. On a single cpu system, without preemption, there is no chance
for the other works to finish, so async_synchronize_full() enters a dead
loop.

It seems async_synchronize_full() wants to synchronize all entries in
all running lists(domains), so maybe we could just check the entry_count
to know whether all works are finished.

Currently, async_synchronize_cookie_domain() expects a non-NULL running
list ( if NULL, there would be NULL pointer dereference ), so maybe a
NULL pointer could be used as an indication for the functions to
synchronize all works in all domains.

Reported-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Li Zhong <zhong@...ux.vnet.ibm.com>
Tested-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Tested-by: Christian Kujau <lists@...dbynature.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Dan Williams <dan.j.williams@...il.com>
Cc: Christian Kujau <lists@...dbynature.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Cong Wang <xiyou.wangcong@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Willy Tarreau <w@....eu>
---
 kernel/async.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/async.c b/kernel/async.c
index 27235f5..397a7c7 100644
--- a/kernel/async.c
+++ b/kernel/async.c
@@ -93,6 +93,13 @@ static async_cookie_t  __lowest_in_progress(struct list_head *running)
 {
 	struct async_entry *entry;
 
+	if (!running) { /* just check the entry count */
+		if (atomic_read(&entry_count))
+			return 0; /* smaller than any cookie */
+		else
+			return next_cookie;
+	}
+
 	if (!list_empty(running)) {
 		entry = list_first_entry(running,
 			struct async_entry, list);
@@ -248,9 +255,7 @@ EXPORT_SYMBOL_GPL(async_schedule_domain);
  */
 void async_synchronize_full(void)
 {
-	do {
-		async_synchronize_cookie(next_cookie);
-	} while (!list_empty(&async_running) || !list_empty(&async_pending));
+	async_synchronize_cookie_domain(next_cookie, NULL);
 }
 EXPORT_SYMBOL_GPL(async_synchronize_full);
 
@@ -270,7 +275,7 @@ EXPORT_SYMBOL_GPL(async_synchronize_full_domain);
 /**
  * async_synchronize_cookie_domain - synchronize asynchronous function calls within a certain domain with cookie checkpointing
  * @cookie: async_cookie_t to use as checkpoint
- * @running: running list to synchronize on
+ * @running: running list to synchronize on, NULL indicates all lists
  *
  * This function waits until all asynchronous function calls for the
  * synchronization domain specified by the running list @list submitted
-- 
1.7.12.2.21.g234cd45.dirty



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