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Subject[PATCH 4/4] workqueue: better define synchronization rule around rescuer->pool updates
From 7bceeff75e5599f5c026797229d897f518dc028e Mon Sep 17 00:00:00 2001
From: Lai Jiangshan <laijs@cn.fujitsu.com>
Date: Tue, 19 Feb 2013 12:17:02 -0800

Rescuers visit different worker_pools to process work items from pools
under pressure. Currently, rescuer->pool is updated outside any
locking and when an outsider looks at a rescuer, there's no way to
tell when and whether rescuer->pool is gonna change. While this
doesn't currently cause any problem, it is nasty.

With recent worker_maybe_bind_and_lock() changes, we can move
rescuer->pool updates inside pool locks such that if rescuer->pool
equals a locked pool, it's guaranteed to stay that way until the pool
is unlocked.

Move rescuer->pool inside pool->lock.

This patch doesn't introduce any visible behavior difference.

tj: Updated the description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
Applied to wq/for-3.10-tmp. Note that I'm gonna reabse the branch
once 3.9-rc1 is out.

Thanks.

kernel/workqueue.c | 3 ++-
kernel/workqueue_internal.h | 1 +
2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 1770b09..0b1e6f2 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2359,8 +2359,8 @@ repeat:
mayday_clear_cpu(cpu, wq->mayday_mask);

/* migrate to the target cpu if possible */
- rescuer->pool = pool;
worker_maybe_bind_and_lock(pool);
+ rescuer->pool = pool;

/*
* Slurp in all works issued via this workqueue and
@@ -2381,6 +2381,7 @@ repeat:
if (keep_working(pool))
wake_up_worker(pool);

+ rescuer->pool = NULL;
spin_unlock_irq(&pool->lock);
}

diff --git a/kernel/workqueue_internal.h b/kernel/workqueue_internal.h
index 0765026..f9c8877 100644
--- a/kernel/workqueue_internal.h
+++ b/kernel/workqueue_internal.h
@@ -32,6 +32,7 @@ struct worker {
struct list_head scheduled; /* L: scheduled works */
struct task_struct *task; /* I: worker task */
struct worker_pool *pool; /* I: the associated pool */
+ /* L: for rescuers */
/* 64 bytes boundary on 64bit, 32 on 32bit */
unsigned long last_active; /* L: last active timestamp */
unsigned int flags; /* X: flags */
--
1.8.1


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