[PATCH 3/4] task_work: deal with task_work callbacks adding more work

[Oleg Nesterov <oleg@redhat.com>]

Shamelessly stolen Al Viro's patch and the changelog, just I didn't
dare to add the proper From tag.

It doesn't matter on normal return to userland path (we'll recheck
the NOTIFY_RESUME flag anyway), but in case of exit_task_work() we'll
need that as soon as we get callbacks capable of triggering more
task_work_add().

Suggested-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/task_work.c |   33 +++++++++++++++++++++------------
 1 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/kernel/task_work.c b/kernel/task_work.c
index b2ff3b7..8dfc48c 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -76,18 +76,27 @@ void task_work_run(void)
 	struct task_struct *task = current;
 	struct task_work *last, *next, *twork;
 
-	raw_spin_lock_irq(&task->pi_lock);
-	last = task->last_twork;
-	task->last_twork = (task->flags & PF_EXITING) ? TWORK_EXITED : NULL;
-	raw_spin_unlock_irq(&task->pi_lock);
+	for (;;) {
+		/*
+		 * twork->func() can do task_work_add(), do not
+		 * set TWORK_EXITED until the list becomes empty.
+		 */
+		raw_spin_lock_irq(&task->pi_lock);
+		last = task->last_twork;
+		if (last)
+			task->last_twork = NULL;
+		else if (task->flags & PF_EXITING)
+			task->last_twork = TWORK_EXITED;
+		raw_spin_unlock_irq(&task->pi_lock);
 
-	if (!last)
-		return;
+		if (!last)
+			return;
 
-	next = last->next;
-	do {
-		twork = next;
-		next = twork->next;
-		twork->func(twork);
-	} while (twork != last);
+		next = last->next;
+		do {
+			twork = next;
+			next = twork->next;
+			twork->func(twork);
+		} while (twork != last);
+	}
 }
-- 
1.5.5.1