linux-kernel - [PATCH 4.9 21/42] signalfd: use wake_up

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Message-Id: <20211213092927.267170805@linuxfoundation.org>
Date:   Mon, 13 Dec 2021 10:30:03 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org, Linus Torvalds" 
        <torvalds@...ux-foundation.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.9 21/42] signalfd: use wake_up_pollfree()

From: Eric Biggers <ebiggers@...gle.com>

commit 9537bae0da1f8d1e2361ab6d0479e8af7824e160 upstream.

wake_up_poll() uses nr_exclusive=1, so it's not guaranteed to wake up
all exclusive waiters.  Yet, POLLFREE *must* wake up all waiters.  epoll
and aio poll are fortunately not affected by this, but it's very
fragile.  Thus, the new function wake_up_pollfree() has been introduced.

Convert signalfd to use wake_up_pollfree().

Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Fixes: d80e731ecab4 ("epoll: introduce POLLFREE to flush ->signalfd_wqh before kfree()")
Cc: stable@...r.kernel.org
Link: https://lore.kernel.org/r/20211209010455.42744-4-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 fs/signalfd.c |   12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -34,17 +34,7 @@
 
 void signalfd_cleanup(struct sighand_struct *sighand)
 {
-	wait_queue_head_t *wqh = &sighand->signalfd_wqh;
-	/*
-	 * The lockless check can race with remove_wait_queue() in progress,
-	 * but in this case its caller should run under rcu_read_lock() and
-	 * sighand_cachep is SLAB_DESTROY_BY_RCU, we can safely return.
-	 */
-	if (likely(!waitqueue_active(wqh)))
-		return;
-
-	/* wait_queue_t->func(POLLFREE) should do remove_wait_queue() */
-	wake_up_poll(wqh, POLLHUP | POLLFREE);
+	wake_up_pollfree(&sighand->signalfd_wqh);
 }
 
 struct signalfd_ctx {