locking/barriers: Convert users of lockless_dereference() to READ_ONCE()

READ_ONCE() now has an implicit smp_read_barrier_depends() call, so it
can be used instead of lockless_dereference() without any change in
semantics.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1508840570-22169-4-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 9d93db8..824a583 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4231,7 +4231,7 @@ static void perf_remove_from_owner(struct perf_event *event)
 	 * indeed free this event, otherwise we need to serialize on
 	 * owner->perf_event_mutex.
 	 */
-	owner = lockless_dereference(event->owner);
+	owner = READ_ONCE(event->owner);
 	if (owner) {
 		/*
 		 * Since delayed_put_task_struct() also drops the last
@@ -4328,7 +4328,7 @@ int perf_event_release_kernel(struct perf_event *event)
 		 * Cannot change, child events are not migrated, see the
 		 * comment with perf_event_ctx_lock_nested().
 		 */
-		ctx = lockless_dereference(child->ctx);
+		ctx = READ_ONCE(child->ctx);
 		/*
 		 * Since child_mutex nests inside ctx::mutex, we must jump
 		 * through hoops. We start by grabbing a reference on the ctx.

kernel/seccomp.c

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 0ae832e..8ac7935 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -189,7 +189,7 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
 	u32 ret = SECCOMP_RET_ALLOW;
 	/* Make sure cross-thread synced filter points somewhere sane. */
 	struct seccomp_filter *f =
-			lockless_dereference(current->seccomp.filter);
+			READ_ONCE(current->seccomp.filter);
 
 	/* Ensure unexpected behavior doesn't result in failing open. */
 	if (unlikely(WARN_ON(f == NULL)))

kernel/task_work.c

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 836a72a..9a9f262 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -67,7 +67,7 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
 	 * we raced with task_work_run(), *pprev == NULL/exited.
 	 */
 	raw_spin_lock_irqsave(&task->pi_lock, flags);
-	while ((work = lockless_dereference(*pprev))) {
+	while ((work = READ_ONCE(*pprev))) {
 		if (work->func != func)
 			pprev = &work->next;
 		else if (cmpxchg(pprev, work, work->next) == work)