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Subject[PATCH wq/for-3.6-fixes 1/2] workqueue: restore POOL_MANAGING_WORKERS
From 552a37e9360a293cd20e7f8ff1fb326a244c5f1e Mon Sep 17 00:00:00 2001
From: Lai Jiangshan <laijs@cn.fujitsu.com>
Date: Mon, 10 Sep 2012 10:03:33 -0700

This patch restores POOL_MANAGING_WORKERS which was replaced by
pool->manager_mutex by 6037315269 "workqueue: use mutex for global_cwq
manager exclusion".

There's a subtle idle worker depletion bug across CPU hotplug events
and we need to distinguish an actual manager and CPU hotplug
preventing management. POOL_MANAGING_WORKERS will be used for the
former and manager_mutex the later.

This patch just lays POOL_MANAGING_WORKERS on top of the existing
manager_mutex and doesn't introduce any synchronization changes. The
next patch will update it.

Note that this patch fixes a non-critical anomaly where
too_many_workers() may return %true spuriously while CPU hotplug is in
progress. While the issue could schedule idle timer spuriously, it
didn't trigger any actual misbehavior.

tj: Rewrote patch description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
Applied to wq/for-3.6-fixes with rewritten patch description to
explain why it's being restored (to fix idle worker depletion across
CPU hotplug).

Thanks!

kernel/workqueue.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index dc7b845..383548e 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -66,6 +66,7 @@ enum {

/* pool flags */
POOL_MANAGE_WORKERS = 1 << 0, /* need to manage workers */
+ POOL_MANAGING_WORKERS = 1 << 1, /* managing workers */

/* worker flags */
WORKER_STARTED = 1 << 0, /* started */
@@ -652,7 +653,7 @@ static bool need_to_manage_workers(struct worker_pool *pool)
/* Do we have too many workers and should some go away? */
static bool too_many_workers(struct worker_pool *pool)
{
- bool managing = mutex_is_locked(&pool->manager_mutex);
+ bool managing = pool->flags & POOL_MANAGING_WORKERS;
int nr_idle = pool->nr_idle + managing; /* manager is considered idle */
int nr_busy = pool->nr_workers - nr_idle;

@@ -1827,6 +1828,7 @@ static bool manage_workers(struct worker *worker)
if (!mutex_trylock(&pool->manager_mutex))
return ret;

+ pool->flags |= POOL_MANAGING_WORKERS;
pool->flags &= ~POOL_MANAGE_WORKERS;

/*
@@ -1836,6 +1838,7 @@ static bool manage_workers(struct worker *worker)
ret |= maybe_destroy_workers(pool);
ret |= maybe_create_worker(pool);

+ pool->flags &= ~POOL_MANAGING_WORKERS;
mutex_unlock(&pool->manager_mutex);
return ret;
}
--
1.7.7.3


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