From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 75A001624D5; Mon, 12 Oct 2026 11:59:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791806364; cv=none; b=jmWfaCbVupZ739g++lykdiw58gG4ymgsDiY7cTZO3JXrZijGDzq7kWposzLHTanfC6FQ2A9e/z1OA5fDJCw0r/YoH7UVVuSor8NaueH/Xndez0/X8zxi64f7By7klesxeKyLPi+ZJGaUNTB1G5wtDz5/hSw9HV5YO5oK4zmcEjY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791806364; c=relaxed/simple; bh=CH9VNDIa4Q76Vk/420L97zwiu9E/kXhYFfhKJsP1dxE=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=OfK8RZlC6U4iuJ7CEdm6FTWoDlWirvFQd/XzgFgN/7l7MNHgwyV4EDVJJqM1W+3ICMopDNCgg0KLvq2r/8+3yStEE0oMX/H/nEWvXRA+j9jMrtsKK9Sj66EIYTrYmKlQTXR3MIVcwa4iAy0rZxdYFCMTkdl8Bj4eQGrI7lgg4j0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SeCdZ44c; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SeCdZ44c" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 701A81F0089B; Mon, 12 Oct 2026 11:59:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791806363; bh=UAz6JeLLovAXng9Kp6ZYxlrWgN0538MMutIGYDPY6CY=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=SeCdZ44cK7G5hQVJJzQKkHXHpS578oBlUXkeldzF4zoXJpLwjtDiN+rfjJUuj/wYj q0g+8TPMYJLMWz/1hBCpSz3NeqXKVUKXh+AT1eqVBMWKXXJjHxiJ9wkQGFeiscgAmr 5bVRQAtGQbQ+DJiUC7QucBm033JFHfgsHjmhgIfYQrjY9JCIUv3TAvVudV9E5TDHN0 b5hUd5nQSjgzlsOIKcvfvd/QUMH0Dj60cBCfvFznsbCCl689SmdnOilAsdmGn7j86o UK837MRfwxGJUGxEPtRG18CyXRNL8riwQirouImuU3DEWJggdgNEkUyuPfIzFbuBRp GUbCTcPUP6dXw== Message-ID: <73b5a5174d98a3ae1dada32684e453debb8dfcd3.camel@kernel.org> Subject: Re: [PATCH v2] fs/fcntl: fix SOFTIRQ-unsafe lock order in fasync signaling From: Jeff Layton To: Oleg Nesterov , w15303746062@163.com, David Laight Cc: chuck.lever@oracle.com, viro@zeniv.linux.org.uk, brauner@kernel.org, alex.aring@gmail.com, jack@suse.cz, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Mingyu Wang <25181214217@stu.xidian.edu.cn> Date: Mon, 12 Oct 2026 07:59:20 -0400 In-Reply-To: References: <20260523135210.590928-1-w15303746062@163.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Sun, 2026-10-11 at 17:57 +0200, Oleg Nesterov wrote: > Nobody replied ;) >=20 > In case it was not clear, I believe the commit 00633c4683828 > ("fs/fcntl: fix SOFTIRQ-unsafe lock order in fasync signaling") > backported to stable trees is wrong. >=20 > Right now the fix is not clear to me, I don't even fully understand > the changelog: >=20 > The deadlock is caused by the rwlock writer fairness mechanism: > 1. CPU 0 (process context) holds read_lock(&tasklist_lock) in do_wait= (). > 2. CPU 1 (process context) attempts write_lock(&tasklist_lock) in > fork() or exit() and spins, which blocks all new readers. >=20 > see below ... >=20 > 3. CPU 0 is interrupted by a softirq (e.g., TCP URG packet reception)= . > 4. The softirq calls send_sigurg() and attempts to acquire > read_lock(&tasklist_lock), deadlocking because CPU 1 is waiting. >=20 > but queued_read_lock_slowpath() does not block the reader if > in_interrupt() =3D=3D T ? >=20 > Mingyu, could you provide more details? How was this problem reported? >=20 > So far I only sent the doc patch to explain that we can't simply avoid ta= sklist: >=20 > [PATCH] pid: document that do_each_pid_task() needs tasklist_lock > https://lore.kernel.org/all/asuaU8H_5MbO3-cW@redhat.com/ >=20 > Oleg. >=20 FYI, I took a closer look, and I think Oleg is correct. When I reviewed this, I missed the fact that the rcu_read_lock() isn't sufficient to stabilize the do_each_pid_task() walk. We probably should revert this patch for now, until the problem and potential solution are more clear. --=20 Jeff Layton