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From:   Siddh Raman Pant <code@siddh.me>
To:     code@siddh.me
Cc:     christophe.jaillet@wanadoo.fr, corbet@lwn.net, dhowells@redhat.com,
        ebiggers@kernel.org, edumazet@google.com,
        linux-kernel-mentees@lists.linuxfoundation.org,
        linux-kernel@vger.kernel.org, mchehab@kernel.org,
        rdunlap@infradead.org
Message-ID: <20220804144152.468916-1-code@siddh.me>
Subject: [PATCH v2 3/3] kernel/watch_queue: Remove wqueue->defunct and use pipe for clear check
Date:   Thu,  4 Aug 2022 20:11:52 +0530
In-Reply-To: <cd2a10ffd8cb11c79754f43d0d20fd3cd4ba9b7e.1659618705.git.code@siddh.me>
References: <cd2a10ffd8cb11c79754f43d0d20fd3cd4ba9b7e.1659618705.git.code@siddh.me>
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The sole use of wqueue->defunct is for checking if the watch queue has
been cleared, but wqueue->pipe is also NULL'd while clearing.

Thus, wqueue->defunct is superfluous, as wqueue->pipe can be checked
for NULL.

Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
Changes in v2:
- The sent patch had accidentally missed removing the member
  from struct, fix that.
- Use !READ_ONCE() instead of =3D=3D NULL, as said by checkpatch.pl.

 include/linux/watch_queue.h |  4 +---
 kernel/watch_queue.c        | 14 +++++---------
 2 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/include/linux/watch_queue.h b/include/linux/watch_queue.h
index c99c39ec6548..88360771c097 100644
--- a/include/linux/watch_queue.h
+++ b/include/linux/watch_queue.h
@@ -55,7 +55,7 @@ struct watch_filter {
  *
  * @rcu: RCU head
  * @filter: Filter on watch_notification::info
- * @pipe: The pipe we're using as a buffer.
+ * @pipe: The pipe we're using as a buffer, NULL when queue is cleared/clo=
sed
  * @watches: Contributory watches
  * @notes: Preallocated notifications
  * @notes_bitmap: Allocation bitmap for notes
@@ -63,7 +63,6 @@ struct watch_filter {
  * @lock: Spinlock
  * @nr_notes: Number of notes
  * @nr_pages: Number of pages in notes[]
- * @defunct: True when queues closed
  */
 struct watch_queue {
 =09struct rcu_head=09=09rcu;
@@ -76,7 +75,6 @@ struct watch_queue {
 =09spinlock_t=09=09lock;
 =09unsigned int=09=09nr_notes;
 =09unsigned int=09=09nr_pages;
-=09bool=09=09=09defunct;
 };
=20
 /**
diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index 8999c4e3076d..825943cf74b2 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -43,7 +43,7 @@ MODULE_LICENSE("GPL");
 static inline bool lock_wqueue(struct watch_queue *wqueue)
 {
 =09spin_lock_bh(&wqueue->lock);
-=09if (unlikely(wqueue->defunct)) {
+=09if (unlikely(!READ_ONCE(wqueue->pipe))) {
 =09=09spin_unlock_bh(&wqueue->lock);
 =09=09return false;
 =09}
@@ -99,15 +99,12 @@ static bool post_one_notification(struct watch_queue *w=
queue,
 =09=09=09=09  struct watch_notification *n)
 {
 =09void *p;
-=09struct pipe_inode_info *pipe =3D READ_ONCE(wqueue->pipe);
+=09struct pipe_inode_info *pipe =3D wqueue->pipe;
 =09struct pipe_buffer *buf;
 =09struct page *page;
 =09unsigned int head, tail, mask, note, offset, len;
 =09bool done =3D false;
=20
-=09if (!pipe)
-=09=09return false;
-
 =09spin_lock_irq(&pipe->rd_wait.lock);
=20
 =09mask =3D pipe->ring_size - 1;
@@ -603,10 +600,9 @@ void watch_queue_clear(struct watch_queue *wqueue)
 =09rcu_read_lock();
 =09spin_lock_bh(&wqueue->lock);
=20
-=09/* Prevent new notifications from being stored. */
-=09wqueue->defunct =3D true;
-
-=09/* This pipe will get freed by caller, and we are anyways clearing. */
+=09/* This pipe will get freed by the caller free_pipe_info().
+=09 * Removing this reference also prevents new notifications.
+=09 */
 =09wqueue->pipe =3D NULL;
=20
 =09while (!hlist_empty(&wqueue->watches)) {
--=20
2.35.1