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 C1488E78499 for ; Sun, 1 Oct 2023 22:32:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234381AbjJAWcs (ORCPT ); Sun, 1 Oct 2023 18:32:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50590 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234325AbjJAWcs (ORCPT ); Sun, 1 Oct 2023 18:32:48 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6E92EC9 for ; Sun, 1 Oct 2023 15:32:45 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 576E2C433C7; Sun, 1 Oct 2023 22:32:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1696199565; bh=v4OVWZcrkE28FR3T1y5fKYkMMBCbE/Aq5nO2NUFR3bU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=fGWv/XvuBf+FJWD/J09e0dzl5IahAw7qSP5aAuBZIPIpETKTLUUj2a4G5U57O6Fnn Hnx4gV+N3Z2/EaUj26czzv4vvuy+n0N7zrxn6YsJ+k4D+wsdeD03VFPne1kHj7NldX t9kB3THxXK/6OOQKSBKL2skJhUJBU+pRajKc6DC/CE4bVH/eFpwwK+SqKwLVe10wq5 /fQPOZof5uQO6CnPMdKTx8UNO2vAZvEi+g7fllEi3UolmhJyRXzTX3N1yzbBn0e27Q tsF4j1G8d8LJ/fH5aZKQS5WY0cvi1dVvWtDs3Rmsh9cveBbd0OV3sosb6L1jnIuazo mARsTvtnDMWUA== Date: Mon, 2 Oct 2023 00:32:41 +0200 From: Frederic Weisbecker To: Neeraj upadhyay Cc: Joel Fernandes , zhuangel570 , paulmck@kernel.org, rcu@vger.kernel.org, josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, like.xu.linux@gmail.com, linussli@tencent.com, foxywang@tencent.com Subject: Re: SRCU: kworker hung in synchronize_srcu Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org Le Sun, Oct 01, 2023 at 07:57:14AM +0530, Neeraj upadhyay a écrit : > > But "more" only checks for CBs in DONE tail. The callbacks which have been just > accelerated are not advanced to DONE tail. > > Having said above, I am still trying to figure out, which callbacks > are actually being pointed > by NEXT tail. Given that __call_srcu() already does a advance and > accelerate, all enqueued > callbacks would be in either WAIT tail (the callbacks for current > active GP) or NEXT_READY > tail (the callbacks for next GP after current one completes). So, they > should already have > GP num assigned and srcu_invoke_callbacks() won't accelerate them. > Only case I can > think of is, if current GP completes after we sample > rcu_seq_current(&ssp->srcu_gp_seq) for > rcu_segcblist_advance() (so, WAIT tail cbs are not moved to DONE tail) > and a new GP is started > before we take snapshot ('s') of next GP for > rcu_segcblist_accelerate(), then the gp num 's' > > gp num of NEXT_READY_TAIL and will be put in NEXT tail. Not sure > if my understanding is correct here. Thoughts? > > __call_srcu() > rcu_segcblist_advance(&sdp->srcu_cblist, > rcu_seq_current(&ssp->srcu_gp_seq)); > s = rcu_seq_snap(&ssp->srcu_gp_seq); > (void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s); Good point! This looks plausible. Does the (buggy) acceleration in srcu_invoke_callbacks() exists solely to handle that case? Because then the acceleration could be moved before the advance on callbacks handling, something like: diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 20d7a238d675..af9d8af1d321 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -1245,6 +1245,11 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp, rcu_segcblist_advance(&sdp->srcu_cblist, rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq)); s = rcu_seq_snap(&ssp->srcu_sup->srcu_gp_seq); + /* + * Acceleration might fail if the preceding call to + * rcu_segcblist_advance() also failed due to a prior incomplete grace + * period. This should be later fixed in srcu_invoke_callbacks(). + */ (void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s); if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) { sdp->srcu_gp_seq_needed = s; @@ -1692,6 +1697,13 @@ static void srcu_invoke_callbacks(struct work_struct *work) ssp = sdp->ssp; rcu_cblist_init(&ready_cbs); spin_lock_irq_rcu_node(sdp); + /* + * Acceleration might have failed in srcu_gp_start_if_needed() if + * the preceding call to rcu_segcblist_advance() also failed due to + * a prior incomplete grace period. + */ + (void)rcu_segcblist_accelerate(&sdp->srcu_cblist, + sdp->srcu_gp_seq_needed); rcu_segcblist_advance(&sdp->srcu_cblist, rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq)); if (sdp->srcu_cblist_invoking || @@ -1720,8 +1732,6 @@ static void srcu_invoke_callbacks(struct work_struct *work) */ spin_lock_irq_rcu_node(sdp); rcu_segcblist_add_len(&sdp->srcu_cblist, -len); - (void)rcu_segcblist_accelerate(&sdp->srcu_cblist, - rcu_seq_snap(&ssp->srcu_sup->srcu_gp_seq)); sdp->srcu_cblist_invoking = false; more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist); spin_unlock_irq_rcu_node(sdp); > > Thanks > Neeraj > > > > if (more) { > > srcu_schedule_cbs_sdp(sdp, 0); > > } > > > > thanks, > > > > - Joel