workqueue: Improve locking rule description for worker fields

* Some worker fields are modified only by the worker itself while holding
  pool->lock thus making them safe to read from self, IRQ context if the CPU
  is running the worker or while holding pool->lock. Add 'K' locking rule
  for them.

* worker->sleeping is currently marked "None" which isn't very descriptive.
  It's used only by the worker itself. Add 'S' locking rule for it.

A future patch will depend on the 'K' rule to access worker->current_* from
the scheduler ticks.
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -126,6 +126,12 @@ enum {
  *    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.
+ *
+ * S: Only modified by worker self.
+ *
  * A: wq_pool_attach_mutex protected.
  *
  * PL: wq_pool_mutex protected.

kernel/workqueue_internal.h

@@ -28,14 +28,15 @@ struct worker {
 		struct hlist_node	hentry;	/* L: while busy */
 	};
 
-	struct work_struct	*current_work;	/* L: work being processed */
-	work_func_t		current_func;	/* L: current_work's fn */
-	struct pool_workqueue	*current_pwq;	/* L: current_work's pwq */
-	unsigned int		current_color;	/* L: current_work's color */
-	int			sleeping;	/* None */
+	struct work_struct	*current_work;	/* K: work being processed and its */
+	work_func_t		current_func;	/* K: function */
+	struct pool_workqueue	*current_pwq;	/* K: pwq */
+	unsigned int		current_color;	/* K: color */
+
+	int			sleeping;	/* S: is worker sleeping? */
 
 	/* used by the scheduler to determine a worker's last known identity */
-	work_func_t		last_func;	/* L: last work's fn */
+	work_func_t		last_func;	/* K: last work's fn */
 
 	struct list_head	scheduled;	/* L: scheduled works */
 
@@ -45,7 +46,7 @@ struct worker {
 	struct list_head	node;		/* A: anchored at pool->workers */
 						/* A: runs through worker->node */
 
-	unsigned long		last_active;	/* L: last active timestamp */
+	unsigned long		last_active;	/* K: last active timestamp */
 	unsigned int		flags;		/* X: flags */
 	int			id;		/* I: worker id */