From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A1B92C433EF for ; Fri, 10 Dec 2021 23:48:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345241AbhLJXwS (ORCPT ); Fri, 10 Dec 2021 18:52:18 -0500 Received: from ams.source.kernel.org ([145.40.68.75]:38900 "EHLO ams.source.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243667AbhLJXwS (ORCPT ); Fri, 10 Dec 2021 18:52:18 -0500 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 7B9D8B82A0D; Fri, 10 Dec 2021 23:48:41 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1861AC00446; Fri, 10 Dec 2021 23:48:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1639180120; bh=Rh0+UYgf7g0HW9AO9oSsZ1PxzAtaaPrEDtqT/JdinS4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=CDJJzXDvlKNisy7vOBuZLhwfmuKP4/mvhf3KF36N6PFa3Y5/MoxlwjOZ69pZ8yzXq a5O4x2DpTZIB3vOvjBBdLjNNpL8yEr6wOVbfIvVYFocL2vVMvl5N4wc11dF6Npc04r wURyt1fgcN7A7t67k4okyM5EEaDWwXAIFhcEsSaSnbv7MQtx870F1qjDesXvPjSLHw J/At/dqyMrMSb27rZbqn9IJxmCOePEo78s7co5IrbmMmgY2w7a5qFVY0odznjbZ1wL a3cwJ8U8gBB7l36RO9qfJxEPDlOrtDUbzHr94STYrDSts+Fy/kpcGxd7jR7u8vedyB KdyX0xJH1f90w== From: Eric Biggers To: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Linus Torvalds Subject: [PATCH 5.10 1/5] wait: add wake_up_pollfree() Date: Fri, 10 Dec 2021 15:48:01 -0800 Message-Id: <20211210234805.39861-2-ebiggers@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20211210234805.39861-1-ebiggers@kernel.org> References: <20211210234805.39861-1-ebiggers@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Eric Biggers commit 42288cb44c4b5fff7653bc392b583a2b8bd6a8c0 upstream. Several ->poll() implementations are special in that they use a waitqueue whose lifetime is the current task, rather than the struct file as is normally the case. This is okay for blocking polls, since a blocking poll occurs within one task; however, non-blocking polls require another solution. This solution is for the queue to be cleared before it is freed, using 'wake_up_poll(wq, EPOLLHUP | POLLFREE);'. However, that has a bug: wake_up_poll() calls __wake_up() with nr_exclusive=1. Therefore, if there are multiple "exclusive" waiters, and the wakeup function for the first one returns a positive value, only that one will be called. That's *not* what's needed for POLLFREE; POLLFREE is special in that it really needs to wake up everyone. Considering the three non-blocking poll systems: - io_uring poll doesn't handle POLLFREE at all, so it is broken anyway. - aio poll is unaffected, since it doesn't support exclusive waits. However, that's fragile, as someone could add this feature later. - epoll doesn't appear to be broken by this, since its wakeup function returns 0 when it sees POLLFREE. But this is fragile. Although there is a workaround (see epoll), it's better to define a function which always sends POLLFREE to all waiters. Add such a function. Also make it verify that the queue really becomes empty after all waiters have been woken up. Reported-by: Linus Torvalds Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20211209010455.42744-2-ebiggers@kernel.org Signed-off-by: Eric Biggers --- include/linux/wait.h | 26 ++++++++++++++++++++++++++ kernel/sched/wait.c | 7 +++++++ 2 files changed, 33 insertions(+) diff --git a/include/linux/wait.h b/include/linux/wait.h index f8b0704968a1e..9b8b0833100a0 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -207,6 +207,7 @@ void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode, void void __wake_up_locked_sync_key(struct wait_queue_head *wq_head, unsigned int mode, void *key); void __wake_up_locked(struct wait_queue_head *wq_head, unsigned int mode, int nr); void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode); +void __wake_up_pollfree(struct wait_queue_head *wq_head); #define wake_up(x) __wake_up(x, TASK_NORMAL, 1, NULL) #define wake_up_nr(x, nr) __wake_up(x, TASK_NORMAL, nr, NULL) @@ -235,6 +236,31 @@ void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode); #define wake_up_interruptible_sync_poll_locked(x, m) \ __wake_up_locked_sync_key((x), TASK_INTERRUPTIBLE, poll_to_key(m)) +/** + * wake_up_pollfree - signal that a polled waitqueue is going away + * @wq_head: the wait queue head + * + * In the very rare cases where a ->poll() implementation uses a waitqueue whose + * lifetime is tied to a task rather than to the 'struct file' being polled, + * this function must be called before the waitqueue is freed so that + * non-blocking polls (e.g. epoll) are notified that the queue is going away. + * + * The caller must also RCU-delay the freeing of the wait_queue_head, e.g. via + * an explicit synchronize_rcu() or call_rcu(), or via SLAB_TYPESAFE_BY_RCU. + */ +static inline void wake_up_pollfree(struct wait_queue_head *wq_head) +{ + /* + * For performance reasons, we don't always take the queue lock here. + * Therefore, we might race with someone removing the last entry from + * the queue, and proceed while they still hold the queue lock. + * However, rcu_read_lock() is required to be held in such cases, so we + * can safely proceed with an RCU-delayed free. + */ + if (waitqueue_active(wq_head)) + __wake_up_pollfree(wq_head); +} + #define ___wait_cond_timeout(condition) \ ({ \ bool __cond = (condition); \ diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c index 21005b980a6b7..a55642aa3f68b 100644 --- a/kernel/sched/wait.c +++ b/kernel/sched/wait.c @@ -223,6 +223,13 @@ void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode) } EXPORT_SYMBOL_GPL(__wake_up_sync); /* For internal use only */ +void __wake_up_pollfree(struct wait_queue_head *wq_head) +{ + __wake_up(wq_head, TASK_NORMAL, 0, poll_to_key(EPOLLHUP | POLLFREE)); + /* POLLFREE must have cleared the queue. */ + WARN_ON_ONCE(waitqueue_active(wq_head)); +} + /* * Note: we use "set_current_state()" _after_ the wait-queue add, * because we need a memory barrier there on SMP, so that any -- 2.34.1