sched/wait: Disambiguate wq_entry->task_list and wq_head->task_list naming

So I've noticed a number of instances where it was not obvious from the
code whether ->task_list was for a wait-queue head or a wait-queue entry.

Furthermore, there's a number of wait-queue users where the lists are
not for 'tasks' but other entities (poll tables, etc.), in which case
the 'task_list' name is actively confusing.

To clear this all up, name the wait-queue head and entry list structure
fields unambiguously:

struct wait_queue_head::task_list => ::head
struct wait_queue_entry::task_list => ::entry

For example, this code:

rqw->wait.task_list.next != &wait->task_list

... is was pretty unclear (to me) what it's doing, while now it's written this way:

rqw->wait.head.next != &wait->entry

... which makes it pretty clear that we are iterating a list until we see the head.

Other examples are:

list_for_each_entry_safe(pos, next, &x->task_list, task_list) {
list_for_each_entry(wq, &fence->wait.task_list, task_list) {

... where it's unclear (to me) what we are iterating, and during review it's
hard to tell whether it's trying to walk a wait-queue entry (which would be
a bug), while now it's written as:

list_for_each_entry_safe(pos, next, &x->head, entry) {
list_for_each_entry(wq, &fence->wait.head, entry) {

Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

block/blk-mq.c

···
 
 	hctx = container_of(wait, struct blk_mq_hw_ctx, dispatch_wait);
 
-	list_del(&wait->task_list);
+	list_del(&wait->entry);
 	clear_bit_unlock(BLK_MQ_S_TAG_WAITING, &hctx->state);
 	blk_mq_run_hw_queue(hctx, true);
 	return 1;

block/blk-wbt.c

···
 	 * in line to be woken up, wait for our turn.
 	 */
 	if (waitqueue_active(&rqw->wait) &&
-	    rqw->wait.task_list.next != &wait->task_list)
+	    rqw->wait.head.next != &wait->entry)
 		return false;
 
 	return atomic_inc_below(&rqw->inflight, get_limit(rwb, rw));

block/kyber-iosched.c

···
 
 	for (i = 0; i < KYBER_NUM_DOMAINS; i++) {
 		INIT_LIST_HEAD(&khd->rqs[i]);
-		INIT_LIST_HEAD(&khd->domain_wait[i].task_list);
+		INIT_LIST_HEAD(&khd->domain_wait[i].entry);
 		atomic_set(&khd->wait_index[i], 0);
 	}
 
···
 {
 	struct blk_mq_hw_ctx *hctx = READ_ONCE(wait->private);
 
-	list_del_init(&wait->task_list);
+	list_del_init(&wait->entry);
 	blk_mq_run_hw_queue(hctx, true);
 	return 1;
 }
···
 	 * run when one becomes available. Note that this is serialized on
 	 * khd->lock, but we still need to be careful about the waker.
 	 */
-	if (list_empty_careful(&wait->task_list)) {
+	if (list_empty_careful(&wait->entry)) {
 		init_waitqueue_func_entry(wait, kyber_domain_wake);
 		wait->private = hctx;
 		ws = sbq_wait_ptr(domain_tokens,
···
 	struct kyber_hctx_data *khd = hctx->sched_data;			\
 	wait_queue_entry_t *wait = &khd->domain_wait[domain];		\
 									\
-	seq_printf(m, "%d\n", !list_empty_careful(&wait->task_list));	\
+	seq_printf(m, "%d\n", !list_empty_careful(&wait->entry));	\
 	return 0;							\
 }
 KYBER_DEBUGFS_DOMAIN_ATTRS(KYBER_READ, read)

drivers/gpu/drm/i915/i915_sw_fence.c

···
 
 	/*
 	 * To prevent unbounded recursion as we traverse the graph of
-	 * i915_sw_fences, we move the task_list from this, the next ready
-	 * fence, to the tail of the original fence's task_list
+	 * i915_sw_fences, we move the entry list from this, the next ready
+	 * fence, to the tail of the original fence's entry list
 	 * (and so added to the list to be woken).
 	 */
 
 	spin_lock_irqsave_nested(&x->lock, flags, 1 + !!continuation);
 	if (continuation) {
-		list_for_each_entry_safe(pos, next, &x->task_list, task_list) {
+		list_for_each_entry_safe(pos, next, &x->head, entry) {
 			if (pos->func == autoremove_wake_function)
 				pos->func(pos, TASK_NORMAL, 0, continuation);
 			else
-				list_move_tail(&pos->task_list, continuation);
+				list_move_tail(&pos->entry, continuation);
 		}
 	} else {
 		LIST_HEAD(extra);
 
 		do {
-			list_for_each_entry_safe(pos, next,
-						 &x->task_list, task_list)
+			list_for_each_entry_safe(pos, next, &x->head, entry)
 				pos->func(pos, TASK_NORMAL, 0, &extra);
 
 			if (list_empty(&extra))
 				break;
 
-			list_splice_tail_init(&extra, &x->task_list);
+			list_splice_tail_init(&extra, &x->head);
 		} while (1);
 	}
 	spin_unlock_irqrestore(&x->lock, flags);
···
 
 static int i915_sw_fence_wake(wait_queue_entry_t *wq, unsigned mode, int flags, void *key)
 {
-	list_del(&wq->task_list);
+	list_del(&wq->entry);
 	__i915_sw_fence_complete(wq->private, key);
 	i915_sw_fence_put(wq->private);
 	if (wq->flags & I915_SW_FENCE_FLAG_ALLOC)
···
 	if (fence == signaler)
 		return true;
 
-	list_for_each_entry(wq, &fence->wait.task_list, task_list) {
+	list_for_each_entry(wq, &fence->wait.head, entry) {
 		if (wq->func != i915_sw_fence_wake)
 			continue;
 
···
 	if (!__test_and_clear_bit(I915_SW_FENCE_CHECKED_BIT, &fence->flags))
 		return;
 
-	list_for_each_entry(wq, &fence->wait.task_list, task_list) {
+	list_for_each_entry(wq, &fence->wait.head, entry) {
 		if (wq->func != i915_sw_fence_wake)
 			continue;
 
···
 		pending |= I915_SW_FENCE_FLAG_ALLOC;
 	}
 
-	INIT_LIST_HEAD(&wq->task_list);
+	INIT_LIST_HEAD(&wq->entry);
 	wq->flags = pending;
 	wq->func = i915_sw_fence_wake;
 	wq->private = i915_sw_fence_get(fence);

drivers/rtc/rtc-imxdi.c

···
 		/*If the write wait queue is empty then there is no pending
 		  operations. It means the interrupt is for DryIce -Security.
 		  IRQ must be returned as none.*/
-		if (list_empty_careful(&imxdi->write_wait.task_list))
+		if (list_empty_careful(&imxdi->write_wait.head))
 			return rc;
 
 		/* DSR_WCF clears itself on DSR read */

fs/cachefiles/rdwr.c

···
 	}
 
 	/* remove from the waitqueue */
-	list_del(&wait->task_list);
+	list_del(&wait->entry);
 
 	/* move onto the action list and queue for FS-Cache thread pool */
 	ASSERT(monitor->op);

fs/eventpoll.c

···
 		 * can't use __remove_wait_queue(). whead->lock is held by
 		 * the caller.
 		 */
-		list_del_init(&wait->task_list);
+		list_del_init(&wait->entry);
 	}
 
 	spin_lock_irqsave(&ep->lock, flags);

fs/fs_pin.c

···
 		rcu_read_unlock();
 		schedule();
 		rcu_read_lock();
-		if (likely(list_empty(&wait.task_list)))
+		if (likely(list_empty(&wait.entry)))
 			break;
 		/* OK, we know p couldn't have been freed yet */
 		spin_lock_irq(&p->wait.lock);

fs/nilfs2/segment.c

···
 	unsigned long flags;
 
 	spin_lock_irqsave(&sci->sc_wait_request.lock, flags);
-	list_for_each_entry_safe(wrq, n, &sci->sc_wait_request.task_list,
-				 wq.task_list) {
+	list_for_each_entry_safe(wrq, n, &sci->sc_wait_request.head, wq.entry) {
 		if (!atomic_read(&wrq->done) &&
 		    nilfs_cnt32_ge(sci->sc_seq_done, wrq->seq)) {
 			wrq->err = err;

fs/orangefs/orangefs-bufmap.c

···
 	spin_lock(&m->q.lock);
 	if (m->c != -1) {
 		for (;;) {
-			if (likely(list_empty(&wait.task_list)))
+			if (likely(list_empty(&wait.entry)))
 				__add_wait_queue_entry_tail(&m->q, &wait);
 			set_current_state(TASK_UNINTERRUPTIBLE);
 
···
 
 	do {
 		long n = left, t;
-		if (likely(list_empty(&wait.task_list)))
+		if (likely(list_empty(&wait.entry)))
 			__add_wait_queue_entry_tail_exclusive(&m->q, &wait);
 		set_current_state(TASK_INTERRUPTIBLE);
 
···
 			left = -EINTR;
 	} while (left > 0);
 
-	if (!list_empty(&wait.task_list))
-		list_del(&wait.task_list);
+	if (!list_empty(&wait.entry))
+		list_del(&wait.entry);
 	else if (left <= 0 && waitqueue_active(&m->q))
 		__wake_up_locked_key(&m->q, TASK_INTERRUPTIBLE, NULL);
 	__set_current_state(TASK_RUNNING);

fs/userfaultfd.c

···
 		 * wouldn't be enough, the smp_mb__before_spinlock is
 		 * enough to avoid an explicit smp_mb() here.
 		 */
-		list_del_init(&wq->task_list);
+		list_del_init(&wq->entry);
 out:
 	return ret;
 }
···
 	 * and it's fine not to block on the spinlock. The uwq on this
 	 * kernel stack can be released after the list_del_init.
 	 */
-	if (!list_empty_careful(&uwq.wq.task_list)) {
+	if (!list_empty_careful(&uwq.wq.entry)) {
 		spin_lock(&ctx->fault_pending_wqh.lock);
 		/*
 		 * No need of list_del_init(), the uwq on the stack
 		 * will be freed shortly anyway.
 		 */
-		list_del(&uwq.wq.task_list);
+		list_del(&uwq.wq.entry);
 		spin_unlock(&ctx->fault_pending_wqh.lock);
 	}
 
···
 	if (!waitqueue_active(wqh))
 		goto out;
 	/* walk in reverse to provide FIFO behavior to read userfaults */
-	wq = list_last_entry(&wqh->task_list, typeof(*wq), task_list);
+	wq = list_last_entry(&wqh->head, typeof(*wq), entry);
 	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
 out:
 	return uwq;
···
 			 * changes __remove_wait_queue() to use
 			 * list_del_init() in turn breaking the
 			 * !list_empty_careful() check in
-			 * handle_userfault(). The uwq->wq.task_list
+			 * handle_userfault(). The uwq->wq.head list
 			 * must never be empty at any time during the
 			 * refile, or the waitqueue could disappear
 			 * from under us. The "wait_queue_head_t"
 			 * parameter of __remove_wait_queue() is unused
 			 * anyway.
 			 */
-			list_del(&uwq->wq.task_list);
+			list_del(&uwq->wq.entry);
 			__add_wait_queue(&ctx->fault_wqh, &uwq->wq);
 
 			write_seqcount_end(&ctx->refile_seq);
···
 				fork_nctx = (struct userfaultfd_ctx *)
 					(unsigned long)
 					uwq->msg.arg.reserved.reserved1;
-				list_move(&uwq->wq.task_list, &fork_event);
+				list_move(&uwq->wq.entry, &fork_event);
 				spin_unlock(&ctx->event_wqh.lock);
 				ret = 0;
 				break;
···
 			if (!list_empty(&fork_event)) {
 				uwq = list_first_entry(&fork_event,
 						       typeof(*uwq),
-						       wq.task_list);
-				list_del(&uwq->wq.task_list);
+						       wq.entry);
+				list_del(&uwq->wq.entry);
 				__add_wait_queue(&ctx->event_wqh, &uwq->wq);
 				userfaultfd_event_complete(ctx, uwq);
 			}
···
 	unsigned long pending = 0, total = 0;
 
 	spin_lock(&ctx->fault_pending_wqh.lock);
-	list_for_each_entry(wq, &ctx->fault_pending_wqh.task_list, task_list) {
+	list_for_each_entry(wq, &ctx->fault_pending_wqh.head, entry) {
 		uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
 		pending++;
 		total++;
 	}
-	list_for_each_entry(wq, &ctx->fault_wqh.task_list, task_list) {
+	list_for_each_entry(wq, &ctx->fault_wqh.head, entry) {
 		uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
 		total++;
 	}

include/linux/wait.h

···
 	unsigned int		flags;
 	void			*private;
 	wait_queue_func_t	func;
-	struct list_head	task_list;
+	struct list_head	entry;
 };
 
 struct wait_queue_head {
 	spinlock_t		lock;
-	struct list_head	task_list;
+	struct list_head	head;
 };
 typedef struct wait_queue_head wait_queue_head_t;
 
···
 #define __WAITQUEUE_INITIALIZER(name, tsk) {					\
 	.private	= tsk,							\
 	.func		= default_wake_function,				\
-	.task_list	= { NULL, NULL } }
+	.entry		= { NULL, NULL } }
 
 #define DECLARE_WAITQUEUE(name, tsk)						\
 	struct wait_queue_entry name = __WAITQUEUE_INITIALIZER(name, tsk)
 
 #define __WAIT_QUEUE_HEAD_INITIALIZER(name) {					\
 	.lock		= __SPIN_LOCK_UNLOCKED(name.lock),			\
-	.task_list	= { &(name).task_list, &(name).task_list } }
+	.head		= { &(name).head, &(name).head } }
 
 #define DECLARE_WAIT_QUEUE_HEAD(name) \
 	struct wait_queue_head name = __WAIT_QUEUE_HEAD_INITIALIZER(name)
···
  */
 static inline int waitqueue_active(struct wait_queue_head *wq_head)
 {
-	return !list_empty(&wq_head->task_list);
+	return !list_empty(&wq_head->head);
 }
 
 /**
···
 
 static inline void __add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 {
-	list_add(&wq_entry->task_list, &wq_head->task_list);
+	list_add(&wq_entry->entry, &wq_head->head);
 }
 
 /*
···
 
 static inline void __add_wait_queue_entry_tail(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 {
-	list_add_tail(&wq_entry->task_list, &wq_head->task_list);
+	list_add_tail(&wq_entry->entry, &wq_head->head);
 }
 
 static inline void
···
 static inline void
 __remove_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 {
-	list_del(&wq_entry->task_list);
+	list_del(&wq_entry->entry);
 }
 
 void __wake_up(struct wait_queue_head *wq_head, unsigned int mode, int nr, void *key);
···
 	struct wait_queue_entry name = {					\
 		.private	= current,					\
 		.func		= function,					\
-		.task_list	= LIST_HEAD_INIT((name).task_list),		\
+		.entry		= LIST_HEAD_INIT((name).entry),			\
 	}
 
 #define DEFINE_WAIT(name) DEFINE_WAIT_FUNC(name, autoremove_wake_function)
···
 	do {									\
 		(wait)->private = current;					\
 		(wait)->func = autoremove_wake_function;			\
-		INIT_LIST_HEAD(&(wait)->task_list);				\
+		INIT_LIST_HEAD(&(wait)->entry);					\
 		(wait)->flags = 0;						\
 	} while (0)
 

include/linux/wait_bit.h

···
 		.wq_entry = {							\
 			.private	= current,				\
 			.func		= wake_bit_function,			\
-			.task_list	=					\
-				LIST_HEAD_INIT((name).wq_entry.task_list),	\
+			.entry		=					\
+				LIST_HEAD_INIT((name).wq_entry.entry),		\
 		},								\
 	}
 

kernel/sched/wait.c

···
 {
 	spin_lock_init(&wq_head->lock);
 	lockdep_set_class_and_name(&wq_head->lock, key, name);
-	INIT_LIST_HEAD(&wq_head->task_list);
+	INIT_LIST_HEAD(&wq_head->head);
 }
 
 EXPORT_SYMBOL(__init_waitqueue_head);
···
 {
 	wait_queue_entry_t *curr, *next;
 
-	list_for_each_entry_safe(curr, next, &wq_head->task_list, task_list) {
+	list_for_each_entry_safe(curr, next, &wq_head->head, entry) {
 		unsigned flags = curr->flags;
 
 		if (curr->func(curr, mode, wake_flags, key) &&
···
 
 	wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
 	spin_lock_irqsave(&wq_head->lock, flags);
-	if (list_empty(&wq_entry->task_list))
+	if (list_empty(&wq_entry->entry))
 		__add_wait_queue(wq_head, wq_entry);
 	set_current_state(state);
 	spin_unlock_irqrestore(&wq_head->lock, flags);
···
 
 	wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
 	spin_lock_irqsave(&wq_head->lock, flags);
-	if (list_empty(&wq_entry->task_list))
+	if (list_empty(&wq_entry->entry))
 		__add_wait_queue_entry_tail(wq_head, wq_entry);
 	set_current_state(state);
 	spin_unlock_irqrestore(&wq_head->lock, flags);
···
 	wq_entry->flags = flags;
 	wq_entry->private = current;
 	wq_entry->func = autoremove_wake_function;
-	INIT_LIST_HEAD(&wq_entry->task_list);
+	INIT_LIST_HEAD(&wq_entry->entry);
 }
 EXPORT_SYMBOL(init_wait_entry);
 
···
 		 * can't see us, it should wake up another exclusive waiter if
 		 * we fail.
 		 */
-		list_del_init(&wq_entry->task_list);
+		list_del_init(&wq_entry->entry);
 		ret = -ERESTARTSYS;
 	} else {
-		if (list_empty(&wq_entry->task_list)) {
+		if (list_empty(&wq_entry->entry)) {
 			if (wq_entry->flags & WQ_FLAG_EXCLUSIVE)
 				__add_wait_queue_entry_tail(wq_head, wq_entry);
 			else
···
  */
 int do_wait_intr(wait_queue_head_t *wq, wait_queue_entry_t *wait)
 {
-	if (likely(list_empty(&wait->task_list)))
+	if (likely(list_empty(&wait->entry)))
 		__add_wait_queue_entry_tail(wq, wait);
 
 	set_current_state(TASK_INTERRUPTIBLE);
···
 
 int do_wait_intr_irq(wait_queue_head_t *wq, wait_queue_entry_t *wait)
 {
-	if (likely(list_empty(&wait->task_list)))
+	if (likely(list_empty(&wait->entry)))
 		__add_wait_queue_entry_tail(wq, wait);
 
 	set_current_state(TASK_INTERRUPTIBLE);
···
 	 *    have _one_ other CPU that looks at or modifies
 	 *    the list).
 	 */
-	if (!list_empty_careful(&wq_entry->task_list)) {
+	if (!list_empty_careful(&wq_entry->entry)) {
 		spin_lock_irqsave(&wq_head->lock, flags);
-		list_del_init(&wq_entry->task_list);
+		list_del_init(&wq_entry->entry);
 		spin_unlock_irqrestore(&wq_head->lock, flags);
 	}
 }
···
 	int ret = default_wake_function(wq_entry, mode, sync, key);
 
 	if (ret)
-		list_del_init(&wq_entry->task_list);
+		list_del_init(&wq_entry->entry);
 	return ret;
 }
 EXPORT_SYMBOL(autoremove_wake_function);

kernel/sched/wait_bit.c

···
 		.wq_entry = {						\
 			.private	= current,			\
 			.func		= wake_atomic_t_function,	\
-			.task_list	=				\
-				LIST_HEAD_INIT((name).wq_entry.task_list), \
+			.entry		=				\
+				LIST_HEAD_INIT((name).wq_entry.entry),	\
 		},							\
 	}
 

mm/filemap.c

···
 	for (;;) {
 		spin_lock_irq(&q->lock);
 
-		if (likely(list_empty(&wait->task_list))) {
+		if (likely(list_empty(&wait->entry))) {
 			if (lock)
 				__add_wait_queue_entry_tail_exclusive(q, wait);
 			else

mm/memcontrol.c

···
 	owait.wait.flags = 0;
 	owait.wait.func = memcg_oom_wake_function;
 	owait.wait.private = current;
-	INIT_LIST_HEAD(&owait.wait.task_list);
+	INIT_LIST_HEAD(&owait.wait.entry);
 
 	prepare_to_wait(&memcg_oom_waitq, &owait.wait, TASK_KILLABLE);
 	mem_cgroup_mark_under_oom(memcg);

mm/shmem.c

···
 static int synchronous_wake_function(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
 {
 	int ret = default_wake_function(wait, mode, sync, key);
-	list_del_init(&wait->task_list);
+	list_del_init(&wait->entry);
 	return ret;
 }
 
···
 		spin_lock(&inode->i_lock);
 		inode->i_private = NULL;
 		wake_up_all(&shmem_falloc_waitq);
-		WARN_ON_ONCE(!list_empty(&shmem_falloc_waitq.task_list));
+		WARN_ON_ONCE(!list_empty(&shmem_falloc_waitq.head));
 		spin_unlock(&inode->i_lock);
 		error = 0;
 		goto out;