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>

2 files changed, 3 insertions, 1 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 09545d445a55..fd9a28a13afd 100644
--- a/kernel/workqueue.c
+++ b/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);
         }
 
diff --git a/kernel/workqueue_internal.h b/kernel/workqueue_internal.h
index 07650264ec15..f9c887731e2b 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 */