linux-kernel - [PATCH] sched: Wake up all non-exclusive waiters in __wake_up

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Date:   Wed, 8 Mar 2017 09:21:52 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     <peterz@...radead.org>, <mingo@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <kernel-team@....com>
Subject: [PATCH] sched: Wake up all non-exclusive waiters in __wake_up_common()

__wake_up_common() should wake up all non-exclusive waiters and
exclusive waiters as many as nr_exclusive, but currently it does not.

Consider a wait queue like the following for example:

   A(exclusive) -> B(non-exclusive) -> C(non-exclusive)

Current code will wake up only A when nr_exclusive = 1, but has to wake
up A, B and C. Make it do as we expect.

Signed-off-by: Byungchul Park <byungchul.park@....com>
---
 kernel/sched/wait.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index 9453efe..0ea1083 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -67,12 +67,23 @@ static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
 {
 	wait_queue_t *curr, *next;
 
+	/*
+	 * We use nr_exclusive = 0 to wake up all no matter whether
+	 * WQ_FLAG_EXCLUSIVE is set. However, we have to distinguish
+	 * between the case and having finished all exclusive wake-up.
+	 * So make nr_exclusive non-zero in advance in the former case.
+	 */
+	nr_exclusive = nr_exclusive ?: -1;
+
 	list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
 		unsigned flags = curr->flags;
 
+		if ((flags & WQ_FLAG_EXCLUSIVE) && !nr_exclusive)
+			continue;
+
 		if (curr->func(curr, mode, wake_flags, key) &&
-				(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
-			break;
+		    (flags & WQ_FLAG_EXCLUSIVE))
+			nr_exclusive--;
 	}
 }
 
-- 
1.9.1