From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.3 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H4, RCVD_IN_MSPIKE_WL autolearn=ham autolearn_force=no version=3.4.4 Received: from second.openwall.net (second.openwall.net [193.110.157.125]) by inbox.vuxu.org (Postfix) with SMTP id F1857215F2 for ; Sat, 10 Aug 2024 06:05:08 +0200 (CEST) Received: (qmail 3996 invoked by uid 550); 10 Aug 2024 04:05:05 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 3961 invoked from network); 10 Aug 2024 04:05:05 -0000 Date: Sat, 10 Aug 2024 00:04:55 -0400 From: Rich Felker To: guolongqiang Cc: "musl@lists.openwall.com" , xufengwei Message-ID: <20240810040455.GG10433@brightrain.aerifal.cx> References: <20240810025125.GD10433@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Subject: [musl] Re: =?utf-8?B?562U5aSN?= =?utf-8?Q?=3A?= [musl] questions about __tl_lock On Sat, Aug 10, 2024 at 03:28:45AM +0000, guolongqiang wrote: > Thank you for explaining. I didn't notice that the parameter of do_futex invoked in mm_release for linux kernel. > > Although the kernel(linux kernel) uses shared option by default to do_futex(wakeup), I think libc can still use > private option to do futex wait, there's no question of correctness. This conclusion comes from the review of > the kernel code. Am I right? > > If that's true, isn't it a matter of us rely on the kernel? You're free to use either in general if the futex word is not being accessed at different addresses referring to the same physical address, but they're different "namespaces" and you have to be consistent which you use for a particular futex -- the wait and wake operations must either both be private or both be non-private, or else the wake will fail to wake the waiter. In the case of the thread list lock, the waker is often linux kernel/fork.c:mm_release, where it performs: put_user(0, tsk->clear_child_tid); do_futex(tsk->clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0, 0); This is a non-private wake, so if we were performing a private wait, we would never wake up. Rich > -----邮件原件----- > 发件人: Rich Felker [mailto:dalias@libc.org] > 发送时间: 2024年8月10日 10:51 > 收件人: guolongqiang > 抄送: musl@lists.openwall.com; xufengwei > 主题: Re: [musl] questions about __tl_lock > > On Fri, Aug 09, 2024 at 03:21:23AM +0000, guolongqiang wrote: > > Hi, all > > I have one question about __tl_lock. The current implementation of __tl_lock shown as follow. > > Obviously __thread_list_lock is a private memory, why don't we pass FUTEX_PRIVATE option to __wait? > > > > ``` > > void __tl_lock(void) > > { > > int tid = __pthread_self()->tid; > > int val = __thread_list_lock; > > if (val == tid) { > > tl_lock_count++; > > return; > > } > > while ((val = a_cas(&__thread_list_lock, 0, tid))) > > __wait(&__thread_list_lock, &tl_lock_waiters, val, 0); } ``` Thank you > > to explain. > > > > The thread list wait operation has to use a non-private futex wait because the wake operation will be performed by the kernel, which performs a non-private wake because that was the original contract from before private futex operations existed. > > Ideally when private waits were added, the kernel exit code path should have been updated to do both private and non-private wakes so that either type of wait would work. But that was overlooked, so even if it were fixed in the kernel now, we couldn't rely on that. > > Rich