From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-4.0 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,RCVD_IN_DNSWL_HI, RCVD_IN_SORBS_SPAM,RP_MATCHES_RCVD shortcircuit=no autolearn=no autolearn_force=no version=3.4.0 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id 13AB7207B9 for ; Tue, 18 Apr 2017 23:18:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758076AbdDRXSh (ORCPT ); Tue, 18 Apr 2017 19:18:37 -0400 Received: from mail-pf0-f180.google.com ([209.85.192.180]:33156 "EHLO mail-pf0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758067AbdDRXS0 (ORCPT ); Tue, 18 Apr 2017 19:18:26 -0400 Received: by mail-pf0-f180.google.com with SMTP id a188so3421833pfa.0 for ; Tue, 18 Apr 2017 16:18:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=from:to:cc:subject:date:message-id:in-reply-to:references; bh=J0SDsN+zqkLsm+rTbC4y7ru2lS0h30r1n+1k5JCz66M=; b=DV6EFiKQgnyk94ifP+3NFn4rsfhAMuHQrkZD5LpCXtFNO5KwRCO7wMt8tPChTOr1KC sxiybtL+PnoAqQi7Hu1Ou6bsxyx1mYi5x53X8VzdEAC095IZ6oUIOG7NgxDBJFniV8y2 VxwZeP/CQcdRCqXgoZpASOGTUZgSYcR9SHx9C10ht1BW+RO4npzfnLWb+SSOYKRQs6rI t3yiUijmc4O5+pUDycqg9FWE1j3LMHQ1GpB+of1lD1KWOFCc1r4tqTcBU6ZvqKqS/7wb yDOBl+tJ8am++d6EKgvbfu+RJLF255mdbJCbwomO4oiFgeivvFKXhl15Fb8jJf0PBBcA ATNQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references; bh=J0SDsN+zqkLsm+rTbC4y7ru2lS0h30r1n+1k5JCz66M=; b=kb2knIDst3769FerjNsWBSBR871/ft49C145QmeqOnjAczxDwj8BXjoRoUmPQFhiz1 njCcnlX9Fg4goce5LsLZQ53eef9bxEtxt+nXjh2WjDipQ/n3rnwrKd9Zvp1rgECY355S ax54R/xUUD8VBQhJIHbwYHad2EtaxXTjX0bvEIQIA71e3sSwUWR5b0NbAM+RlKQA/d+q sXAJn9+aF0vM/L+SRbighcMQt6ZyCrzz8OA8W/4Jo1RIS2qwfU9o+9VLZUsnVg8WUVwC q4GwTxqTw3qJPKaLqIPo04ZYXjyRX/y2CmqVcPj8tLYoymIXjJlizE9G7gPQS53dFB6Z ViJQ== X-Gm-Message-State: AN3rC/4W1snjtstOW9RGdwEA+ihUTTXe0yI/BJY7vd9OK2i0pruGxmnt ubzU5YHAbrWRLj61 X-Received: by 10.84.215.16 with SMTP id k16mr6157983pli.58.1492557506082; Tue, 18 Apr 2017 16:18:26 -0700 (PDT) Received: from roshar.svl.corp.google.com ([100.96.218.30]) by smtp.gmail.com with ESMTPSA id z21sm492764pfk.95.2017.04.18.16.18.24 (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 18 Apr 2017 16:18:25 -0700 (PDT) From: Brandon Williams To: git@vger.kernel.org Cc: Brandon Williams , e@80x24.org, jrnieder@gmail.com Subject: [PATCH v5 10/11] run-command: add note about forking and threading Date: Tue, 18 Apr 2017 16:18:04 -0700 Message-Id: <20170418231805.61835-11-bmwill@google.com> X-Mailer: git-send-email 2.12.2.816.g2cccc81164-goog In-Reply-To: <20170418231805.61835-1-bmwill@google.com> References: <20170417220818.44917-1-bmwill@google.com> <20170418231805.61835-1-bmwill@google.com> Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org All non-Async-Signal-Safe functions (e.g. malloc and die) were removed between 'fork' and 'exec' in start_command in order to avoid potential deadlocking when forking while multiple threads are running. This deadlocking is possible when a thread (other than the one forking) has acquired a lock and didn't get around to releasing it before the fork. This leaves the lock in a locked state in the resulting process with no hope of it ever being released. Add a note describing this potential pitfall before the call to 'fork()' so people working in this section of the code know to only use Async-Signal-Safe functions in the child process. Signed-off-by: Brandon Williams --- run-command.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/run-command.c b/run-command.c index 615b6e9c9..df1edd963 100644 --- a/run-command.c +++ b/run-command.c @@ -537,6 +537,15 @@ int start_command(struct child_process *cmd) prepare_cmd(&argv, cmd); childenv = prep_childenv(cmd->env); + /* + * NOTE: In order to prevent deadlocking when using threads special + * care should be taken with the function calls made in between the + * fork() and exec() calls. No calls should be made to functions which + * require acquiring a lock (e.g. malloc) as the lock could have been + * held by another thread at the time of forking, causing the lock to + * never be released in the child process. This means only + * Async-Signal-Safe functions are permitted in the child. + */ cmd->pid = fork(); failed_errno = errno; if (!cmd->pid) { -- 2.12.2.816.g2cccc81164-goog