From patchwork Sat Dec 11 00:02:21 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Eric Biggers X-Patchwork-Id: 523377 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 B2DD7C433F5 for ; Sat, 11 Dec 2021 00:02:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243597AbhLKAGG (ORCPT ); Fri, 10 Dec 2021 19:06:06 -0500 Received: from ams.source.kernel.org ([145.40.68.75]:44858 "EHLO ams.source.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230472AbhLKAGG (ORCPT ); Fri, 10 Dec 2021 19:06:06 -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 933EAB82A48; Sat, 11 Dec 2021 00:02:29 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 29209C341C6; Sat, 11 Dec 2021 00:02:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1639180948; bh=NG/2rpNIoOuXW0kEaBE8DZNMg4kHewm20HOlMHfvuCs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=oXUoubL5+ktTr5WJspzo2i70HQmdYm9ejp2eqeX+KJ2MDmPMXAmDzfZoL6Ub66GjZ Sn0ZmcDsT2UwQgJ8+9ap/qUP8cWziT7Crd81zM+6KPyvY4tb1oWINO6WPxmB9qMzS7 d/POfQOpRD5+SEa3aWL6zNmFOdc6sLi+GMHVe98BgCkesNcxf+WjK74SrLS+DRvEJP x3usIa8cklFpQX0hl51mndVBublrYLPffx5a8kDw+6VL7A9Zon9FeSsOXIK7K8gcUx Cx8+iplHT2TrhPczujxrvEZlkmaqpN3rRTp4Auofv2Tx99RbTCoAqBbdA9p0pfw0I0 Ks8m7S42aGYCQ== From: Eric Biggers To: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Linus Torvalds Subject: [PATCH 4.14 1/3] wait: add wake_up_pollfree() Date: Fri, 10 Dec 2021 16:02:21 -0800 Message-Id: <20211211000223.50630-2-ebiggers@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20211211000223.50630-1-ebiggers@kernel.org> References: <20211211000223.50630-1-ebiggers@kernel.org> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: stable@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 158715445ffb6..42e648083610e 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -191,6 +191,7 @@ void __wake_up_locked_key_bookmark(struct wait_queue_head *wq_head, void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode, int nr, 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, int nr); +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) @@ -215,6 +216,31 @@ void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode, int nr); #define wake_up_interruptible_sync_poll(x, m) \ __wake_up_sync_key((x), TASK_INTERRUPTIBLE, 1, (void *) (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 e296084643821..82f4c8f216f8a 100644 --- a/kernel/sched/wait.c +++ b/kernel/sched/wait.c @@ -213,6 +213,13 @@ void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode, int nr_e } 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(POLLHUP | 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