workqueue: better define synchronization rule around rescuer->pool updates

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>

kernel/workqueue.c

@@ -2357,8 +2357,8 @@ 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
@@ -2379,6 +2379,7 @@ repeat:
 		if (keep_working(pool))
 			wake_up_worker(pool);
 
+		rescuer->pool = NULL;
 		spin_unlock_irq(&pool->lock);
 	}
 

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 */