From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from simark.ca by simark.ca with LMTP id 29AHJqsJkl/sTAAAWB0awg (envelope-from ) for ; Thu, 22 Oct 2020 18:37:31 -0400 Received: by simark.ca (Postfix, from userid 112) id 93A2E1E89B; Thu, 22 Oct 2020 18:37:31 -0400 (EDT) X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on simark.ca X-Spam-Level: X-Spam-Status: No, score=-1.1 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,MAILING_LIST_MULTI,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.2 Received: from lists.lttng.org (lists.lttng.org [167.114.26.123]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by simark.ca (Postfix) with ESMTPS id AAE011E58E for ; Thu, 22 Oct 2020 18:37:30 -0400 (EDT) Received: from lists-lttng01.efficios.com (localhost [IPv6:::1]) by lists.lttng.org (Postfix) with ESMTP id 4CHMgZ10gHz15fk; Thu, 22 Oct 2020 18:37:30 -0400 (EDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=lists.lttng.org; s=default; t=1603406250; bh=lb9XhfwyawphElhKebFAREY5480mP4Nkf70fMxqba3c=; h=Date:To:Cc:Subject:List-Id:List-Unsubscribe:List-Archive: List-Post:List-Help:List-Subscribe:From:Reply-To:From; b=CW0Be6BqmlBhhd60OlhA08bETrU1pv2VzJot1YoAPUPzAb9TrDXF/SIJqA0GQ8Crg ykTa7Z3DivYLLcR8HaH7RBWb1s2yzr71rcNkjZ+t1LFtC7ubne45e+WrWU64b2Eu/M GHm//dBfhxMNEEGFHUA/Qsnjw/HHmUeRXdy8KAvAls8Jr0EFTIv0eb8j8WOJYVesiP qgzBPs69oAftaKRcNBiOnEjtjFUGOI6BkbNZW9XrxJtgKzZz8rlzTev4YMGaYKhTzo SjFzOS4l45L9VA59M4bkR/3j6uOi3+p8HWJYTm3kuQv2XkoKgKTRKG3SOJmIOekZuj sN47PuS6XF+Mw== Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by lists.lttng.org (Postfix) with ESMTPS id 4CHMgX2dSnz15sm for ; Thu, 22 Oct 2020 18:37:28 -0400 (EDT) Received: from paulmck-ThinkPad-P72.home (50-39-104-11.bvtn.or.frontiernet.net [50.39.104.11]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 7BCF324650; Thu, 22 Oct 2020 22:30:21 +0000 (UTC) Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 26F2F3522AB9; Thu, 22 Oct 2020 15:30:21 -0700 (PDT) Date: Thu, 22 Oct 2020 15:30:21 -0700 To: mathieu.desnoyers@efficios.com Cc: stephen@networkplumber.org, stern@rowland.harvard.edu, jiangshanlai@gmail.com, lttng-dev@lists.lttng.org, linux-kernel@vger.kernel.org Message-ID: <20201022223021.GA8535@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Disposition: inline User-Agent: Mutt/1.9.4 (2018-02-28) Subject: [lttng-dev] [PATCH] call_rcu: Fix race between rcu_barrier() and call_rcu_data_free() X-BeenThere: lttng-dev@lists.lttng.org X-Mailman-Version: 2.1.31 Precedence: list List-Id: LTTng development list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: "Paul E. McKenney via lttng-dev" Reply-To: paulmck@kernel.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: lttng-dev-bounces@lists.lttng.org Sender: "lttng-dev" The current code can lose RCU callbacks at shutdown time, which can result in hangs. This lossage can happen as follows: o A thread invokes call_rcu_data_free(), which executes up through the wake_call_rcu_thread(). At this point, the call_rcu_data structure has been drained of callbacks, but is still on the call_rcu_data_list. Note that this thread does not hold the call_rcu_mutex. o Another thread invokes rcu_barrier(), which traverses the call_rcu_data_list under the protection of call_rcu_mutex, a list which still includes the above newly drained structure. This thread therefore adds a callback to the newly drained call_rcu_data structure. It then releases call_rcu_mutex and enters a mystifying loop that does futex stuff. o The first thread finishes executing call_rcu_data_free(), which acquires call_rcu_mutex just long enough to remove the newly drained call_rcu_data structure from call_rcu_data_list. Which causes one of the rcu_barrier() invocation's callbacks to be leaked. o The second thread's rcu_barrier() invocation never returns resulting in a hang. This commit therefore changes call_rcu_data_free() to acquire call_rcu_mutex before checking the call_rcu_data structure for callbacks. In the case where there are no callbacks, call_rcu_mutex is held across both the check and the removal from call_rcu_data_list, thus preventing rcu_barrier() from adding a callback in the meantime. In the case where there are callbacks, call_rcu_mutex must be momentarily dropped across the call to get_default_call_rcu_data(), which can itself acquire call_rcu_mutex. This momentary drop is not a problem because any callbacks that rcu_barrier() might queue during that period of time will be moved to the default call_rcu_data structure, and the lock will be held across the full time including moving those callbacks and removing the call_rcu_data structure that was passed into call_rcu_data_free() from call_rcu_data_list. With this fix, a several-hundred-CPU test successfully completes more than 5,000 executions. Without this fix, it fails within a few tens of executions. Although the failures happen more quickly on larger systems, in theory this could happen on a single-CPU system, courtesy of preemption. Signed-off-by: Paul E. McKenney Cc: Mathieu Desnoyers Cc: Stephen Hemminger Cc: Alan Stern Cc: Lai Jiangshan Cc: Cc: --- urcu-call-rcu-impl.h | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/src/urcu-call-rcu-impl.h b/src/urcu-call-rcu-impl.h index b6ec6ba..18fd65a 100644 --- a/src/urcu-call-rcu-impl.h +++ b/src/urcu-call-rcu-impl.h @@ -772,9 +772,13 @@ void call_rcu_data_free(struct call_rcu_data *crdp) while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == 0) (void) poll(NULL, 0, 1); } + call_rcu_lock(&call_rcu_mutex); if (!cds_wfcq_empty(&crdp->cbs_head, &crdp->cbs_tail)) { - /* Create default call rcu data if need be */ + call_rcu_unlock(&call_rcu_mutex); + /* Create default call rcu data if need be. */ + /* CBs queued here will be handed to the default list. */ (void) get_default_call_rcu_data(); + call_rcu_lock(&call_rcu_mutex); __cds_wfcq_splice_blocking(&default_call_rcu_data->cbs_head, &default_call_rcu_data->cbs_tail, &crdp->cbs_head, &crdp->cbs_tail); @@ -783,7 +787,6 @@ void call_rcu_data_free(struct call_rcu_data *crdp) wake_call_rcu_thread(default_call_rcu_data); } - call_rcu_lock(&call_rcu_mutex); cds_list_del(&crdp->list); call_rcu_unlock(&call_rcu_mutex); _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev