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 2A5A0C433FE for ; Mon, 28 Nov 2022 10:51:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230408AbiK1KvI (ORCPT ); Mon, 28 Nov 2022 05:51:08 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46428 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230422AbiK1KvD (ORCPT ); Mon, 28 Nov 2022 05:51:03 -0500 Received: from metanate.com (unknown [IPv6:2001:8b0:1628:5005::111]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 14BF165C5 for ; Mon, 28 Nov 2022 02:50:57 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=metanate.com; s=stronger; h=In-Reply-To:Content-Type:References:Message-ID: Subject:To:From:Date:Reply-To:Cc:Content-Transfer-Encoding:Content-ID: Content-Description; bh=rkH4EDzFj5GJH/oZZwg9Xi75afujMR0gQt8dEe9YYLI=; b=StGmS K0ahOhPDsedu2XVpS4qA7mxW25pQt+Bj/H+sLEbWNdphHaKe28AHetDF8ZJ1iKnLFbRCGhbunG/1P nEtII5K2CEyCD4mlLcjO9W0KbwjDg5rHmwQ52KHUGN9tc1N/6fngh/sm44wzWBQpj+R5IPjLTFS3W tmri3aBeI+dUvg7ziKOE5MCMXSigSWLPFt/5l3716ZnbyWq5L+hHAU0gVhfkP7luhv3P3OJhST/Xb TkpK4km2FmmcrE45m6LYSLlKKgYHm1rgehEZS+FQltP0zHJyxnPCk6mjrFeEdD8tLNzGqmuUePnMO 9XrKFSURFeau9RZOKs2/pkXc080ow==; Received: from [81.174.171.191] (helo=donbot) by email.metanate.com with esmtpsa (TLS1.3) tls TLS_AES_256_GCM_SHA384 (Exim 4.95) (envelope-from ) id 1ozbiv-0004Ry-79; Mon, 28 Nov 2022 10:50:53 +0000 Date: Mon, 28 Nov 2022 10:50:52 +0000 From: John Keeping To: alsa-devel@alsa-project.org, linux-kernel@vger.kernel.org, Takashi Iwai Subject: Re: [PATCH] ALSA: pcm: fix tracing reason in hw_ptr_error Message-ID: References: <20221125162327.297440-1-john@metanate.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Authenticated: YES Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Nov 26, 2022 at 12:51:53PM +0900, Takashi Sakamoto wrote: > Hi, > > On Fri, Nov 25, 2022 at 04:23:26PM +0000, John Keeping wrote: > > Strings need to be specially marked in trace events to ensure the > > content is captured, othewise the trace just shows the value of the > > pointer. > > > > Signed-off-by: John Keeping > > --- > > sound/core/pcm_trace.h | 6 +++--- > > 1 file changed, 3 insertions(+), 3 deletions(-) > > Thanks for the patch, while I have a slight concern about the intension > of change. > > Let's see message in below commit to add 'trace_safe_str()' in > 'kernel/trace/trace.c': > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9a6944fee68 > > "The proper way to handle displaying any string that is not guaranteed to be > in the kernel core rodata section, is to copy it into the ring buffer via > the __string(), __assign_str() and __get_str() helper macros." > > Additionally, the macros looks to be alias to __dynamic_array() or so. > > In our case, the pointer of 'why' argument should points to two > hardcoded strings; "hw_ptr skipping" and "Unexpected hw_ptr". As long as > I know, they are put into any of .rodata section. > > ``` > $ objdump -s sound/core/snd-pcm.ko -j .rodata.str1.1 > ``` > 0830 74757265 20436861 6e6e656c 204d6170 ture Channel Map > 0840 00585255 4e3a2025 730a0055 6e657870 .XRUN: %s..Unexp > 0850 65637465 64206877 5f707472 0068775f ected hw_ptr.hw_ > 0860 70747220 736b6970 70696e67 004c6f73 ptr skipping.Los > 0870 7420696e 74657272 75707473 3f00756e t interrupts?.un > ``` > > I think current implementation is enough safe. Thanks for this analysis - I hadn't looked into the history of this. It seems that trace-cmd's record/report functionality does not support rodata strings in this way though. Without this patch, the trace log is not very informative: $ trace-cmd record -e snd_pcm:hw_ptr_error ^C $ trace-cmd report irq/49-ehci_hcd-111 [002] 65.785147: hw_ptr_error: pcmC1D0p/sub0: ERROR: c0b1b3c7 With this patch applied this becomes: irq/49-ehci_hcd-111 [002] 435.758754: hw_ptr_error: pcmC2D0p/sub0: ERROR: Lost interrupts? > Nevertheless, explicit usage of the macros are developer friendly in my > opinion since string buffer in C language tends to bring problems and > the usage of macro would reduce careless future mistakes. In the case, > when probing the event, the string is copied to memory for event > structure, thus it adds slight overhead than current implementation. I > leave the decision to maintainer. > > To maintainer, if you apply the patch to your tree, feel free to add my > review tag. > > Reviewed-by: Takashi Sakamoto > > > diff --git a/sound/core/pcm_trace.h b/sound/core/pcm_trace.h > > index f18da2050772..350b40b906ca 100644 > > --- a/sound/core/pcm_trace.h > > +++ b/sound/core/pcm_trace.h > > @@ -88,19 +88,19 @@ TRACE_EVENT(hw_ptr_error, > > __field( unsigned int, device ) > > __field( unsigned int, number ) > > __field( unsigned int, stream ) > > - __field( const char *, reason ) > > + __string( reason, why ) > > ), > > TP_fast_assign( > > __entry->card = (substream)->pcm->card->number; > > __entry->device = (substream)->pcm->device; > > __entry->number = (substream)->number; > > __entry->stream = (substream)->stream; > > - __entry->reason = (why); > > + __assign_str(reason, why); > > ), > > TP_printk("pcmC%dD%d%s/sub%d: ERROR: %s", > > __entry->card, __entry->device, > > __entry->stream == SNDRV_PCM_STREAM_PLAYBACK ? "p" : "c", > > - __entry->number, __entry->reason) > > + __entry->number, __get_str(reason)) > > ); > > > > TRACE_EVENT(applptr, > > -- > > 2.38.1