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 A89412C181 for ; Sat, 25 Jul 2026 15:22:24 +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=1784992945; cv=none; b=It8clCljtdMjlyCN+j6ZLaKmRk4qCQ0/WI6PUbqRBaHlbiQ2sUj5js7lSPHvV/+SfGunO+QlYi1BBUD4XMPtIL9Lko/CqTpn64WUGM+vGFCcLtq/2vASwEtM2fdfJp0ERuYqi+2DtFIOiMhUSL5Nx8d+e8NkT1d5TWhzwon8uHU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784992945; c=relaxed/simple; bh=UDA8sxsFrJc6kse0pjsO7q6KXTTXFlEyg3CupuSNzk4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=MgLDH6IP8Hc2NyqQQYq4dmHNsHUosfFYfSbGe/d8MrLi6JhmNgG5dwM/980y9VS6R8LdDsCniZxREPtVAEBTirdX1Q1szrhRUNQYq+7ap1ni4EF4mJ766Cm/aboYoxAEo9PEP2lWC1aHiGFjycVPzUXgd0cxs9DsjSK8fmc2Pqw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EHyEtjeJ; 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="EHyEtjeJ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D1BFA1F000E9; Sat, 25 Jul 2026 15:22:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784992944; bh=z7QWoHe4SRU3kL+//2y2tkGQOyHEaXgIVxBkqDdiZJ8=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=EHyEtjeJi1GUFW76pGAmm+q0SGeh+5OHxueHnw48XJ02srFC1i58C+7O/Aa/QRCrv QBRMpgpH9imvIv6sVlQx9ugwyRiB8T00QtStUtpbgPodE4fvVNLIvkDYJaeux4q1b+ l/3m8dh9U9j+0jP2dggJ829/AfFNwjpMFdysGUwGaWXsHfN8D7S7JAeJPz2mjMqmZ2 6NNqJ/wXbBi7p8Ajb2l0js231tqn/ohN037LkJZCBbpTsNNrdp9+yXO1Tb6qn9c2Nb UISXxWhCKBDnlciFte+yXNq91+CsTSb51UInL2rGyeayUOpG62VNxoNSGCQGSx59Ab Uu5DengdqGUVA== From: Sasha Levin To: stable@vger.kernel.org Cc: Breno Leitao , Dominique Martinet , Sasha Levin Subject: [PATCH 5.15.y] 9p: skip nlink update in cacheless mode to fix WARN_ON Date: Sat, 25 Jul 2026 11:22:21 -0400 Message-ID: <20260725152221.3856854-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <2026072033-zesty-lustily-01bf@gregkh> References: <2026072033-zesty-lustily-01bf@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Breno Leitao [ Upstream commit 574aa0b4799470ac814479f1138d19efe6262255 ] v9fs_dec_count() unconditionally calls drop_nlink() on regular files, even when the inode's nlink is already zero. In cacheless mode the client refetches inode metadata from the server (the source of truth) on every operation, so by the time v9fs_remove() returns, the locally cached nlink may already reflect the post-unlink value: 1. Client initiates unlink, server processes it and sets nlink to 0 2. Client refetches inode metadata (nlink=0) before unlink returns 3. Client's v9fs_remove() completes successfully 4. Client calls v9fs_dec_count() which calls drop_nlink() on nlink=0 This race is easily triggered under heavy unlink workloads, such as stress-ng's unlink stressor, producing the following warning: WARNING: fs/inode.c:417 at drop_nlink+0x4c/0xc8 Call trace: drop_nlink+0x4c/0xc8 v9fs_remove+0x1e0/0x250 [9p] v9fs_vfs_unlink+0x20/0x38 [9p] vfs_unlink+0x13c/0x258 ... In cacheless mode the server is authoritative and the inode is on its way out, so locally adjusting nlink buys nothing. Skip v9fs_dec_count() entirely when neither CACHE_META nor CACHE_LOOSE is set, which both avoids the warning and removes a class of nlink races (two concurrent unlinkers observing nlink > 0 and both calling drop_nlink()) that an nlink == 0 guard alone would only narrow rather than close. Fixes: ac89b2ef9b55 ("9p: don't maintain dir i_nlink if the exported fs doesn't either") Cc: stable@vger.kernel.org Suggested-by: Dominique Martinet Signed-off-by: Breno Leitao Message-ID: <20260421-9p-v2-1-48762d294fad@debian.org> Signed-off-by: Dominique Martinet [ Adapted `v9ses->cache & (CACHE_META | CACHE_LOOSE)` bitmask test to the pre-bitmask exclusive enum form `v9ses->cache != CACHE_LOOSE && v9ses->cache != CACHE_FSCACHE`. ] Signed-off-by: Sasha Levin --- fs/9p/vfs_inode.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c index ef103ef392ee3a..2caea3390d6a51 100644 --- a/fs/9p/vfs_inode.c +++ b/fs/9p/vfs_inode.c @@ -521,10 +521,19 @@ static int v9fs_at_to_dotl_flags(int flags) * - ext4 (with dir_nlink feature enabled) sets nlink to 1 if a dir has more * than EXT4_LINK_MAX (65000) links. * + * In cacheless mode the server is the source of truth for nlink and the + * inode is going away immediately, so locally adjusting i_nlink buys + * nothing and races with concurrent metadata fetches that may already + * have observed the post-unlink value (nlink == 0). + * * @inode: inode whose nlink is being dropped */ static void v9fs_dec_count(struct inode *inode) { + struct v9fs_session_info *v9ses = v9fs_inode2v9ses(inode); + + if (v9ses->cache != CACHE_LOOSE && v9ses->cache != CACHE_FSCACHE) + return; if (!S_ISDIR(inode->i_mode) || inode->i_nlink > 2) drop_nlink(inode); } -- 2.53.0