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=-23.4 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, URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=unavailable 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 6409FC11F68 for ; Wed, 30 Jun 2021 22:30:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4AE3861476 for ; Wed, 30 Jun 2021 22:30:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233131AbhF3Wcl (ORCPT ); Wed, 30 Jun 2021 18:32:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48754 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232459AbhF3Wci (ORCPT ); Wed, 30 Jun 2021 18:32:38 -0400 Received: from mail-qt1-x832.google.com (mail-qt1-x832.google.com [IPv6:2607:f8b0:4864:20::832]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 64D16C0617A8 for ; Wed, 30 Jun 2021 15:30:08 -0700 (PDT) Received: by mail-qt1-x832.google.com with SMTP id w13so2781692qtc.0 for ; Wed, 30 Jun 2021 15:30:08 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=Hiptwy9TUvWyDXIfvAtOM6Kej8lazuA4fp4IHXgFEhA=; b=l0phQZTqmoJDi9w/25alZe0CUOgvyrerAYN4ksIjN3XQzYhjFywzQQM9zLWORpjAHY TR/bgReRORJbKHhGgEy3CRtXOMHFbyS2fvJH69GKHNPOjrF58lBBGX/5o7lNtvZA8Yau XrPJEba8gk0NpHC3uBuNjlyrVQME5lCs3bgODUU5QVhZOoyR8NPCOuEFbPgT3GYfCMYy ARXPYd/u+oeYXlIA0SpGqGJtFCUkTEYfGPZQGrfllgrGqI+j8FM0CiPaW2qMfEKO3FXZ Lm4/MgZCo/QeYjY2F9A8gdhdqYJ+Lu6NQ1ZEz7M7dcBZxWUGg7kSjAFNZEhTwNJGvRys WjFA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=Hiptwy9TUvWyDXIfvAtOM6Kej8lazuA4fp4IHXgFEhA=; b=K8QvgOipQY41aRX6lQIumbrsPzgSTano3UYXXHRpKYaZWnNAXz+51ARQn8ZN7jL0Wu YQQ4wC90lXXPupLPqDTRsbfdoGU+P9w3tl198zwtllt+PeL9UPkUgyfl1AK7Flrx/yIa 3KJ3kzDhHlajMNehu+cSmp5oosu+KXHTDP/7HUvzVQWL8E6okKOI0piY9TkmT24BixmJ xHPF4h/FmqRfNyim6GPYQzrMvO7ajzJz4I/F2uli/Y3jfZT85WVIRcl42rzuPTvmqYqT 4Gd+nvz/x2ydUVuhS8/J3HY4AmP952cuec5aRjOFFwNJqZDakf50Q94L/oTiRot13o6Q 5/GQ== X-Gm-Message-State: AOAM530IoNIOAz6nB5Yyj4LaIIL+hc/dX1+NwXVVk4tzuW36vFefZyPX AQJwKgx4Qr1VsFs7MtOXJh94VaiFTe1MaH2HWnV5xg== X-Google-Smtp-Source: ABdhPJxaKRUyPs4V1DSgIvmJq4TKnBpdyjCHx41DHlWJ5jxaPeUqodvfc4mqeHL+zXsZzsJQvDEkpuOiY4kYLWBnAG8= X-Received: by 2002:a05:622a:316:: with SMTP id q22mr33689033qtw.153.1625092206907; Wed, 30 Jun 2021 15:30:06 -0700 (PDT) MIME-Version: 1.0 References: <20210630003406.4013668-1-paulburton@google.com> In-Reply-To: <20210630003406.4013668-1-paulburton@google.com> From: Joel Fernandes Date: Wed, 30 Jun 2021 18:29:55 -0400 Message-ID: Subject: Re: [PATCH 1/2] tracing: Simplify & fix saved_tgids logic To: Paul Burton Cc: linux-kernel@vger.kernel.org, Steven Rostedt , Ingo Molnar , stable@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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). 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 >