From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E6C3EC14D for ; Wed, 28 Jun 2023 21:34:10 +0000 (UTC) Received: from mga02.intel.com (mga02.intel.com [134.134.136.20]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 78B671FE4; Wed, 28 Jun 2023 14:34:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1687988049; x=1719524049; h=from:to:cc:subject:in-reply-to:references:date: message-id:mime-version; bh=2zFRPcPqSdw/rQupEM6m4agW4/QKfyjYyUa1kit0f1s=; b=UIa1fHs2Zs9fdB87YCNRFAuxiWbmg/365ZNADpyUTOAtzHvLgfO5lBUK wxw67CFmygXg+ogUUMNSmnnNLywwAJ0Au4bjzmidC/nbeyA6beV653ui/ lSYP1SiJBy5+Qb4mDjF7U7L2U0i/tqr98wyd7l0MXuKC+xLb3kZz6wVk/ 4nCnAFdauRuLIoNlwLs9RgXmrPzHLU9plxZuBFcUBTmly0A3B2lUc86t9 D2mOHA42J2FRRrzQU0rztrUyjmIDZ8kycv95abvSgM7JDW6P8UmVR+wzU xTd3Fit49RtYOAk49V48l7GcxjvapcCmElyyuz4oLkqLm/WUSy1+cpXET g==; X-IronPort-AV: E=McAfee;i="6600,9927,10755"; a="351758361" X-IronPort-AV: E=Sophos;i="6.01,166,1684825200"; d="scan'208";a="351758361" Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by orsmga101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 28 Jun 2023 14:34:08 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6600,9927,10755"; a="787172958" X-IronPort-AV: E=Sophos;i="6.01,166,1684825200"; d="scan'208";a="787172958" Received: from vcostago-desk1.jf.intel.com (HELO vcostago-desk1) ([10.54.70.17]) by fmsmga004-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 28 Jun 2023 14:34:07 -0700 From: Vinicius Costa Gomes To: Florian Kauer , Jesse Brandeburg , Tony Nguyen , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Vedang Patel , Maciej Fijalkowski , Jithu Joseph , Andre Guedes , Simon Horman Cc: netdev@vger.kernel.org, kurt@linutronix.de, intel-wired-lan@lists.osuosl.org, linux-kernel@vger.kernel.org Subject: Re: [Intel-wired-lan] [PATCH net v2] igc: Prevent garbled TX queue with XDP ZEROCOPY In-Reply-To: <20230628091148.62256-1-florian.kauer@linutronix.de> References: <20230628091148.62256-1-florian.kauer@linutronix.de> Date: Wed, 28 Jun 2023 14:34:07 -0700 Message-ID: <87a5wjqnjk.fsf@intel.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain X-Spam-Status: No, score=-4.4 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED, RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_NONE, T_SCC_BODY_TEXT_LINE,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Florian Kauer writes: > In normal operation, each populated queue item has > next_to_watch pointing to the last TX desc of the packet, > while each cleaned item has it set to 0. In particular, > next_to_use that points to the next (necessarily clean) > item to use has next_to_watch set to 0. > > When the TX queue is used both by an application using > AF_XDP with ZEROCOPY as well as a second non-XDP application > generating high traffic, the queue pointers can get in > an invalid state where next_to_use points to an item > where next_to_watch is NOT set to 0. > > However, the implementation assumes at several places > that this is never the case, so if it does hold, > bad things happen. In particular, within the loop inside > of igc_clean_tx_irq(), next_to_clean can overtake next_to_use. > Finally, this prevents any further transmission via > this queue and it never gets unblocked or signaled. > Secondly, if the queue is in this garbled state, > the inner loop of igc_clean_tx_ring() will never terminate, > completely hogging a CPU core. > > The reason is that igc_xdp_xmit_zc() reads next_to_use > before acquiring the lock, and writing it back > (potentially unmodified) later. If it got modified > before locking, the outdated next_to_use is written > pointing to an item that was already used elsewhere > (and thus next_to_watch got written). > > Fixes: 9acf59a752d4 ("igc: Enable TX via AF_XDP zero-copy") > Signed-off-by: Florian Kauer > Reviewed-by: Kurt Kanzenbach > Tested-by: Kurt Kanzenbach > --- This patch doesn't directly apply because there's a small conflict with commit 95b681485563 ("igc: Avoid transmit queue timeout for XDP"), but really easy to solve. Anyway, good catch: Acked-by: Vinicius Costa Gomes Cheers, -- Vinicius