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 X-Spam-Level: X-Spam-Status: No, score=-23.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_IN_DEF_DKIM_WL autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 731F3C11F64 for ; Thu, 1 Jul 2021 17:32:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4C49261409 for ; Thu, 1 Jul 2021 17:32:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232593AbhGARei (ORCPT ); Thu, 1 Jul 2021 13:34:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46202 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230139AbhGAReg (ORCPT ); Thu, 1 Jul 2021 13:34:36 -0400 Received: from mail-pj1-x1030.google.com (mail-pj1-x1030.google.com [IPv6:2607:f8b0:4864:20::1030]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 31B3EC061762 for ; Thu, 1 Jul 2021 10:32:05 -0700 (PDT) Received: by mail-pj1-x1030.google.com with SMTP id g6-20020a17090adac6b029015d1a9a6f1aso6242193pjx.1 for ; Thu, 01 Jul 2021 10:32:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=BbJUiUUe8T9b2PK6TM2EJq7HBKzquI9DW1+aUliaiPo=; b=CVBp7SxkahiNSc/6amQlsQrupBIvPGzMuT+qFHMb8J+fEgPmJ+jSNCObUz3UPwYwDh 96U0UYYZnKH2aWE3M+lvXivMBhfaVSK+gjY5ur0I4ocShvUctW+KnLGvoFzWfrQslIk6 qhd/uIEmUd5jSNYfWvz65ylCrTmu0PcSVOlxbZrg1xHtLdgXfIko/5vmpmtm3li2QbnP Y0moswHricRpt+NVr5AWm0QclZ+uXuMmIJKEWZ9wiXx/T/+XpZgl7Z/SMudxcLfqBcNP TTnrGYRIDdk4Nea4OFwd9s1bwBi4xFDWbYXNUIWUbl0VP7GQ9CetjiSW63t0JNc8CgD3 wD7Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=BbJUiUUe8T9b2PK6TM2EJq7HBKzquI9DW1+aUliaiPo=; b=ArYl+hU2bd4ckRk5iHGSUWaYHBbUu22yDS0AISuu1gi/w1hs5mTnXKiefIhlNZy9gK OiDNuk3YYm76JCBE1eIm7Kq950HxmX1ApUdwcyu5S6HZv6wCf8v11Bv5N73k+/62Og7Q ucjS/QlUEZhbBZc29wrQ7E5QFyjs/fLzZlomipxVVWYmSt+2Er9majbtJPaYavdEOjzJ DemStY3QlCF6jJN9l2estPmConkZgjjgJEfuwpmaB2/5Sw/hrgXB1DiaaSg1+GivqBP6 vrbFmN/bFelUO6tZRgpJcVJEdaiFcw9Tsd1F2MXRTz5UM6iHMY74OgHUexZ8KW99wCQT J6MA== X-Gm-Message-State: AOAM5334L5hrQfUr1rvwsCXJcE6CwIgmn3v9v4ENaV/LH2R146CVHjsx TSOLD2GjVut1eWtmjRI7NFnknvtEKlZpTA== X-Google-Smtp-Source: ABdhPJwaRu6TdRHiOXUsrbUGWmqyGU/c31E1IgwClXw1IEA4MbKdQrnpni+2qvPFr+yjvDGvNmfw+Q== X-Received: by 2002:a17:902:8503:b029:127:8abc:191c with SMTP id bj3-20020a1709028503b02901278abc191cmr845772plb.21.1625160724477; Thu, 01 Jul 2021 10:32:04 -0700 (PDT) Received: from google.com ([2620:15c:280:201:558a:406a:d453:dbe5]) by smtp.gmail.com with ESMTPSA id y13sm444667pgp.16.2021.07.01.10.32.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 01 Jul 2021 10:32:04 -0700 (PDT) Date: Thu, 1 Jul 2021 10:31:59 -0700 From: Paul Burton To: Joel Fernandes Cc: linux-kernel@vger.kernel.org, Steven Rostedt , Ingo Molnar , stable@vger.kernel.org Subject: Re: [PATCH 1/2] tracing: Simplify & fix saved_tgids logic Message-ID: References: <20210630003406.4013668-1-paulburton@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org Hi Joel, On Wed, Jun 30, 2021 at 06:29:55PM -0400, Joel Fernandes wrote: > On Tue, Jun 29, 2021 at 8:34 PM Paul Burton wrote: > > > > The tgid_map array records a mapping from pid to tgid, where the index > > of an entry within the array is the pid & the value stored at that index > > is the tgid. > > > > The saved_tgids_next() function iterates over pointers into the tgid_map > > array & dereferences the pointers which results in the tgid, but then it > > passes that dereferenced value to trace_find_tgid() which treats it as a > > pid & does a further lookup within the tgid_map array. It seems likely > > that the intent here was to skip over entries in tgid_map for which the > > recorded tgid is zero, but instead we end up skipping over entries for > > which the thread group leader hasn't yet had its own tgid recorded in > > tgid_map. > > > > A minimal fix would be to remove the call to trace_find_tgid, turning: > > > > if (trace_find_tgid(*ptr)) > > > > into: > > > > if (*ptr) > > > > ..but it seems like this logic can be much simpler if we simply let > > seq_read() iterate over the whole tgid_map array & filter out empty > > entries by returning SEQ_SKIP from saved_tgids_show(). Here we take that > > approach, removing the incorrect logic here entirely. > > Looks reasonable except for one nit: > > > Signed-off-by: Paul Burton > > Fixes: d914ba37d714 ("tracing: Add support for recording tgid of tasks") > > Cc: Steven Rostedt > > Cc: Ingo Molnar > > Cc: Joel Fernandes > > Cc: > > --- > > kernel/trace/trace.c | 38 +++++++++++++------------------------- > > 1 file changed, 13 insertions(+), 25 deletions(-) > > > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > > index d23a09d3eb37b..9570667310bcc 100644 > > --- a/kernel/trace/trace.c > > +++ b/kernel/trace/trace.c > > @@ -5608,37 +5608,20 @@ static const struct file_operations tracing_readme_fops = { > > > > static void *saved_tgids_next(struct seq_file *m, void *v, loff_t *pos) > > { > > - int *ptr = v; > > + int pid = ++(*pos); > > > > - if (*pos || m->count) > > - ptr++; > > - > > - (*pos)++; > > - > > - for (; ptr <= &tgid_map[PID_MAX_DEFAULT]; ptr++) { > > - if (trace_find_tgid(*ptr)) > > - return ptr; > > It would be great if you can add back the check for !tgid_map to both > next() and show() as well, for added robustness (since the old code > previously did it). That condition cannot happen, because both next() & show() are called to iterate through the content of the seq_file & by definition their v argument is non-NULL (else seq_file would have finished iterating already). That argument came from either start() or an earlier call to next(), which would only have returned a non-NULL pointer into tgid_map if tgid_map is non-NULL. I've added comments to this effect in v2, though the second patch in v2 does wind up effectively adding back the check in next() anyway in order to reuse some code. I was tempted to just add the redundant checks anyway (pick your battles and all) but for show() in particular it wound up making things seem non-sensical to me ("display the value describing this non-NULL pointer into tgid_map only if tgid_map is not NULL?"). Thanks, Paul > With that change: > Reviewed-by: Joel Fernandes (Google) > > thanks, > > -Joel > > > > - } > > + if (pid > PID_MAX_DEFAULT) > > + return NULL; > > > > - return NULL; > > + return &tgid_map[pid]; > > } > > > > static void *saved_tgids_start(struct seq_file *m, loff_t *pos) > > { > > - void *v; > > - loff_t l = 0; > > - > > - if (!tgid_map) > > + if (!tgid_map || *pos > PID_MAX_DEFAULT) > > return NULL; > > > > - v = &tgid_map[0]; > > - while (l <= *pos) { > > - v = saved_tgids_next(m, v, &l); > > - if (!v) > > - return NULL; > > - } > > - > > - return v; > > + return &tgid_map[*pos]; > > } > > > > static void saved_tgids_stop(struct seq_file *m, void *v) > > @@ -5647,9 +5630,14 @@ static void saved_tgids_stop(struct seq_file *m, void *v) > > > > static int saved_tgids_show(struct seq_file *m, void *v) > > { > > - int pid = (int *)v - tgid_map; > > + int *entry = (int *)v; > > + int pid = entry - tgid_map; > > + int tgid = *entry; > > + > > + if (tgid == 0) > > + return SEQ_SKIP; > > > > - seq_printf(m, "%d %d\n", pid, trace_find_tgid(pid)); > > + seq_printf(m, "%d %d\n", pid, tgid); > > return 0; > > } > > > > > > base-commit: 62fb9874f5da54fdb243003b386128037319b219 > > -- > > 2.32.0.93.g670b81a890-goog > >