[ 055/108] Fix a dead loop in async_synchronize_full()

Ben Hutchings
2012-10-07T23:31:50Z

From: Li Zhong <zhong@linux.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@linux.vnet.ibm.com>
Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Christian Kujau <lists@nerdbynature.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dan Williams <dan.j.williams@gmail.com>
Cc: Christian Kujau <lists@nerdbynature.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
kernel/async.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)

--- a/kernel/async.c
+++ b/kernel/async.c
@@ -87,6 +87,13 @@ static async_cookie_t __lowest_in_progr
{
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);
@@ -236,9 +243,7 @@ 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);

@@ -258,7 +263,7 @@ EXPORT_SYMBOL_GPL(async_synchronize_full
/**
* 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