From mboxrd@z Thu Jan 1 00:00:00 1970 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=MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 14751 invoked from network); 11 Jan 2024 16:59:25 -0000 Received: from second.openwall.net (193.110.157.125) by inbox.vuxu.org with ESMTPUTF8; 11 Jan 2024 16:59:25 -0000 Received: (qmail 5903 invoked by uid 550); 11 Jan 2024 16:57:48 -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 5869 invoked from network); 11 Jan 2024 16:57:47 -0000 Date: Thu, 11 Jan 2024 11:59:26 -0500 From: Rich Felker To: libc-alpha@sourceware.org, musl@lists.openwall.com, linux-api@vger.kernel.org Message-ID: <20240111165926.GT4163@brightrain.aerifal.cx> References: <20240111164620.GS4163@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240111164620.GS4163@brightrain.aerifal.cx> User-Agent: Mutt/1.5.21 (2010-09-15) Subject: [musl] Re: Robust mutex ABI problem (kernel docs regression) On Thu, Jan 11, 2024 at 11:46:21AM -0500, Rich Felker wrote: > It came to my attention while reviewing some proposals for additional > error checking in musl libc that the kernel folks introduced a quiet > regression in the futex ABI for robust mutexes. > > Prior to kernel commit 9c40365a65d62d7c06a95fb331b3442cb02d2fd9, bit > 29 of the futex lock word was reserved, meaning it could never be part > of a TID. This allowed both glibc and musl to use special values like > (glibc): > > /* Magic cookie representing robust mutex with dead owner. */ > #define PTHREAD_MUTEX_INCONSISTENT INT_MAX > /* Magic cookie representing not recoverable robust mutex. */ > #define PTHREAD_MUTEX_NOTRECOVERABLE (INT_MAX - 1) > > to represent special states needed for robust mutex consistency > handling, without the risk that, when masked with FUTEX_TID_MASK > (0x3fffffff), they could be equal to the TID of a real task, which > could result in the kernel robustlist-processing mishandling them. On further investigation, it looks like glibc is not using these directly in the futex word but keeping them in a separate owner field protected by the lock. I'll try to follow up with what it actually is doing, but I don't see a way, without at least having one reserved value in the low 30 bits (FUTEX_TID_MASK), to represent a lock that cannot be taken by any task. I'm guessing glibc does some dance of actually taking the lock then releasing it in this case..? On musl we keep the entire state in the lock word so that it's all atomic. Rich