linux-kernel - [PATCH 03/34] watch_queue: use atomic find_bit() in post_one

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Date:   Sat, 18 Nov 2023 07:50:34 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org,
        Christian Brauner <brauner@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Siddh Raman Pant <code@...dh.me>,
        Yury Norov <yury.norov@...il.com>,
        Dave Airlie <airlied@...hat.com>,
        David Disseldorp <ddiss@...e.de>,
        Philipp Stanner <pstanner@...hat.com>,
        Nick Alcock <nick.alcock@...cle.com>
Cc:     Jan Kara <jack@...e.cz>,
        Mirsad Todorovac <mirsad.todorovac@....unizg.hr>,
        Matthew Wilcox <willy@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Alexey Klimov <klimov.linux@...il.com>
Subject: [PATCH 03/34] watch_queue: use atomic find_bit() in post_one_notification()

post_one_notification() searches for a set bit in wqueue->notes_bitmap,
and after some housekeeping work clears it, firing a BUG() if someone
else cleared the bit in-between.

We can allocate a bit atomically with the atomic find_and_clear_bit(),
and remove the BUG() possibility entirely.

Signed-off-by: Yury Norov <yury.norov@...il.com>
---
 kernel/watch_queue.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index 778b4056700f..07edd4a2b463 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -112,7 +112,7 @@ static bool post_one_notification(struct watch_queue *wqueue,
 	if (pipe_full(head, tail, pipe->ring_size))
 		goto lost;
 
-	note = find_first_bit(wqueue->notes_bitmap, wqueue->nr_notes);
+	note = find_and_clear_bit(wqueue->notes_bitmap, wqueue->nr_notes);
 	if (note >= wqueue->nr_notes)
 		goto lost;
 
@@ -133,10 +133,6 @@ static bool post_one_notification(struct watch_queue *wqueue,
 	buf->flags = PIPE_BUF_FLAG_WHOLE;
 	smp_store_release(&pipe->head, head + 1); /* vs pipe_read() */
 
-	if (!test_and_clear_bit(note, wqueue->notes_bitmap)) {
-		spin_unlock_irq(&pipe->rd_wait.lock);
-		BUG();
-	}
 	wake_up_interruptible_sync_poll_locked(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
 	done = true;
 
-- 
2.39.2