From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=216.40.44.14 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794297141; b=3ygovBQ8tKszFwu8Ci6/Ir0TecOwbcFwsfXho+JeJllQwQ7tKYAjlY19zPXNxWw8nI/q yKx4YbZ8IZx365lxLKGE9koTkqK6MRPhCtGsArFy3EyFtJCc1GTPWEpIH68NuuO0NwPA4 KX9BjeVNcNGan/J0rcwnEenspYpKru+tsgc/lSluXa4laT7XEn6sxCZFAcjx5r3x2cQ8z bV6bQPR/gTaqAQc4TqR5vwexpEwf8ZSMd613ZV85GjZ/GNtewSiB+C1G4Dm82pg+RSiSM HxJPp7rTfnsseJtQEwcYqPPXxHyGZb2HtSVdAT+7aqYX2ULbNfGFyzmvMGzjO1zcMtg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794297141; h=DMARC-Filter:Received:Received:Date:From:To:Cc:Subject:Message-ID: In-Reply-To:References:X-Mailer:MIME-Version:Content-Type: Content-Transfer-Encoding:X-Rspamd-Server:X-Rspamd-Queue-Id: X-Stat-Signature:X-Spam-Status:X-Session-Marker:X-Session-ID: DKIM-Signature:X-HE-Tag:X-HE-Meta; bh=HOR1+kqQhwJITF9mrUMDzf7ZzPSccks+bMhcYnxMQ/k=; b=PeGn8Ho6Q/S8GWzZSWGQfXdPDZjAp02QMTXJ6KXD9UzQjLcldDfcgTtAP7kI4z07PXAk dQT83PDF5EYvA1c+VKdHs+qCLJzf5XNybKDA91mpYaU6qHqep6wTWYxRr5CX6++ZLmxdG ewiJCXa2tLnZT8Qu2J32H5VZKKtrpptbDoicDaDbvXqDnUiB1trA93OkTEEliSn2PeLVi fnfJ0EQYSXnm3W63FTEbnqjFxa4/U1/QptpQuMkjzk2Xsn0bFznbO0BFmBPLWPk4vmvAg gUGxucemTURr7kh1umB/r8rmNwCGmqlxC7Sd0vEjoEcZHiTUKcSApFX/jIQ0EcOAOTg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=goodmis.org; spf=pass smtp.mailfrom=goodmis.org; dkim=pass header.d=goodmis.org header.i=@goodmis.org header.a=rsa-sha256 header.s=dkim1 header.b=Wo0WadD1; arc=none smtp.remote-ip=216.40.44.14 Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D1B41606A334; Tue, 10 Nov 2026 07:52:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org D1B41606A334 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=goodmis.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=goodmis.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=goodmis.org header.i=@goodmis.org header.a=rsa-sha256 header.s=dkim1 header.b=Wo0WadD1 Received: from omf04.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 6959A140503; Tue, 10 Nov 2026 07:52:12 +0000 (UTC) Received: from [HIDDEN] (Authenticated sender: rostedt@goodmis.org) by omf04.hostedemail.com (Postfix) with ESMTPA id 1BA702002B; Tue, 10 Nov 2026 07:52:07 +0000 (UTC) Date: Tue, 10 Nov 2026 02:52:02 -0500 From: Steven Rostedt To: Vincent Donnefort Cc: Breno Leitao , Masami Hiramatsu , Mathieu Desnoyers , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Steven Rostedt , kernel-team@meta.com, stable@vger.kernel.org Subject: Re: [PATCH v2] ring-buffer: Handle a reader taking the new head in rb_handle_head_page() Message-ID: <20261110025202.78c8c03a@fedora> In-Reply-To: References: <20261105-ring-buffer-head-moved-v2-1-05da40cebccc@debian.org> <20261105155729.0f9c7e4d@gandalf.local.home> X-Mailer: Claws Mail 4.4.0 (GTK 3.24.52; x86_64-redhat-linux-gnu) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspamout05 X-Rspamd-Queue-Id: 1BA702002B X-Stat-Signature: erqs333doy343w6acayxmgdjtkgouj9d X-Session-Marker: 726F737465647440676F6F646D69732E6F7267 X-Session-ID: U2FsdGVkX1+YQOdajhCDf1ZjZQfnHpfSD5rLF0zgf5s= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=goodmis.org; h=date:from:to:cc:subject:message-id:in-reply-to:references:mime-version:content-type:content-transfer-encoding; s=dkim1; bh=HOR1+kqQhwJITF9mrUMDzf7ZzPSccks+bMhcYnxMQ/k=; b=Wo0WadD1kJznyFIlA6jRp+DI7L2vl/XdShuuFurlVUnOZW8enSWCf4tTmX8XqPYhIYO+74AuaTtTUfTsxtk6eyVW9RZrG29QbG9n/6kOUVsBN48xeHTGhpLh3F8SHLikyRWKwLMOjIyzUX2hJlUozCfOqyvc3ya0n8MGTvAebyQ= X-HE-Tag: 1794297127-995168 X-HE-Meta: U2FsdGVkX1+hTXjhiPB/SOj4L/XPStP7ECk2j9J4x/CsE0t8QgKNpm9izosUDHdljRIJTbLF4rRiA/aFukUGls7Y9P9meRaZDRHrmTc/xDDJdHrsuzRqcTU+YviHj0930OyYBGYD6NOguoAf1ebrXF1wnnqJ5356UB0WiNN+M6paW1SRvUgubYwabm7Po6e/6/zL0gkjPvzJ3QRW9zQPr8VOzhr/U5mcSJpcXaynm6/RKPuj8L5Db6hBNIX/85+RESUCtwW3UShT7Zrw9bTGeC3ViAdQxqVQDCfrmKq4a5ZvF1R6X7nAL6b5EQyTRdAvknI1oj35S5XMUGHaQFBvUYl52H0jLw/1 On Mon, 9 Nov 2026 09:21:32 +0000 Vincent Donnefort wrote: > > The issue I think you hit was that we have two HEAD pages, when interrupts > > come in. That is, we can get a state of: > > > > --> subbuf --UPDATE--> subbuf --> subbuf --HEAD--> subbuf --> > > How we can end-up in this situation? The cmpxch will only swap HEAD for UPDATE. The cpu_buffer->head_page is only updated by the reader, when it did its last read. Let's look at it one at a time: cpu_buffer->head_page tail | v v +--> subbuf ----> subbuf --HEAD--> subbuf ----> subbuf ----> subbuf --+ | | +---------------------------------------------------------------------+ Then the writer moves the HEAD page around the buffer while there are no reads happening cpu_buffer->head_page tail | v v +--> subbuf ----> subbuf ----> subbuf --HEAD--> subbuf ----> subbuf --+ | | +---------------------------------------------------------------------+ cpu_buffer->head_page | tail v v +--> subbuf ----> subbuf ----> subbuf ----> subbuf --HEAD--> subbuf --+ | | +---------------------------------------------------------------------+ cpu_buffer->head_page | tail v v +-HEAD-> subbuf ----> subbuf ----> subbuf ----> subbuf ----> subbuf --+ | | +---------------------------------------------------------------------+ cpu_buffer->head_page tail | v v +--> subbuf --HEAD--> subbuf ----> subbuf ----> subbuf ----> subbuf --+ | | +---------------------------------------------------------------------+ Now, I'll break up the update part: cpu_buffer->head_page tail | v v +--> subbuf --UPDATE--> subbuf ----> subbuf ----> subbuf ----> subbuf --+ | | +-----------------------------------------------------------------------+ cpu_buffer->head_page tail | v v +--> subbuf --UPDATE--> subbuf --HEAD--> subbuf ----> subbuf ----> subbuf --+ | | +---------------------------------------------------------------------------+ An interrupt comes in and moves the head even more. cpu_buffer->head_page tail | v v +--> subbuf --UPDATE--> subbuf ----> subbuf --HEAD--> subbuf ----> subbuf --+ | | +---------------------------------------------------------------------------+ Now the reader decides to read again and starts between the UPDATE and the HEAD cpu_buffer->head_page tail | v v +--> subbuf --UPDATE--> subbuf ----> subbuf --HEAD--> subbuf ----> subbuf --+ | | +---------------------------------------------------------------------------+ It finds a page with the HEAD pointer set but not the UPDATE pointer and does a swap! reader_page -----+ | cpu_buffer->head_page | tail | | v v v +--> subbuf --UPDATE--> subbuf ----> subbuf ----> subbuf --HEAD--> subbuf --+ | | +---------------------------------------------------------------------------+ Now the commit finishes and moves forward and will even hit a blank page but the last write is now on the reader page. This breaks the entire integrity of the ring buffer. Does this make sense? -- Steve