workqueue: Drop the special locking rule for worker->flags and worker_pool->flags

worker->flags used to be accessed from scheduler hooks without grabbing
pool->lock for concurrency management. This is no longer true since
6d25be57 ("sched/core, workqueues: Distangle worker accounting from rq
lock"). Also, it's unclear why worker_pool->flags was using the "X" rule.
All relevant users are accessing it under the pool lock.

Let's drop the special "X" rule and use the "L" rule for these flag fields
instead. While at it, replace the CONTEXT comment with
lockdep_assert_held().

This allows worker_set/clr_flags() to be used from context which isn't the
worker itself. This will be used later to implement assinging work items to
workers before waking them up so that workqueue can have better control over
which worker executes which work item on which CPU.

The only actual changes are sanity checks. There shouldn't be any visible
behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -122,11 +122,6 @@ enum {
  *
  * L: pool->lock protected.  Access with pool->lock held.
  *
- * X: During normal operation, modification requires pool->lock and should
- *    be done only from local cpu.  Either disabling preemption on local
- *    cpu or grabbing pool->lock is enough for read access.  If
- *    POOL_DISASSOCIATED is set, it's identical to L.
- *
  * K: Only modified by worker while holding pool->lock. Can be safely read by
  *    self, while holding pool->lock or from IRQ context if %current is the
  *    kworker.
@@ -160,7 +155,7 @@ struct worker_pool {
 	int			cpu;		/* I: the associated cpu */
 	int			node;		/* I: the associated node ID */
 	int			id;		/* I: pool ID */
-	unsigned int		flags;		/* X: flags */
+	unsigned int		flags;		/* L: flags */
 
 	unsigned long		watchdog_ts;	/* L: watchdog timestamp */
 	bool			cpu_stall;	/* WD: stalled cpu bound pool */
@@ -910,15 +905,12 @@ static void wake_up_worker(struct worker_pool *pool)
  * @flags: flags to set
  *
  * Set @flags in @worker->flags and adjust nr_running accordingly.
- *
- * CONTEXT:
- * raw_spin_lock_irq(pool->lock)
  */
 static inline void worker_set_flags(struct worker *worker, unsigned int flags)
 {
 	struct worker_pool *pool = worker->pool;
 
-	WARN_ON_ONCE(worker->task != current);
+	lockdep_assert_held(&pool->lock);
 
 	/* If transitioning into NOT_RUNNING, adjust nr_running. */
 	if ((flags & WORKER_NOT_RUNNING) &&
@@ -935,16 +927,13 @@ static inline void worker_set_flags(struct worker *worker, unsigned int flags)
  * @flags: flags to clear
  *
  * Clear @flags in @worker->flags and adjust nr_running accordingly.
- *
- * CONTEXT:
- * raw_spin_lock_irq(pool->lock)
  */
 static inline void worker_clr_flags(struct worker *worker, unsigned int flags)
 {
 	struct worker_pool *pool = worker->pool;
 	unsigned int oflags = worker->flags;
 
-	WARN_ON_ONCE(worker->task != current);
+	lockdep_assert_held(&pool->lock);
 
 	worker->flags &= ~flags;
 

kernel/workqueue_internal.h

@@ -48,7 +48,7 @@ struct worker {
 						/* A: runs through worker->node */
 
 	unsigned long		last_active;	/* K: last active timestamp */
-	unsigned int		flags;		/* X: flags */
+	unsigned int		flags;		/* L: flags */
 	int			id;		/* I: worker id */
 
 	/*