From nobody Thu Feb 12 02:59:36 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D87ECC7EE24 for ; Sat, 29 Apr 2023 07:03:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347281AbjD2HDf (ORCPT ); Sat, 29 Apr 2023 03:03:35 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42336 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230298AbjD2HDM (ORCPT ); Sat, 29 Apr 2023 03:03:12 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4B0852D55; Sat, 29 Apr 2023 00:03:11 -0700 (PDT) Date: Sat, 29 Apr 2023 07:03:07 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1682751787; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=SDaA+xUryUCZ8I28N9aEWarm2HCgNFHRBUqq4ZipNkw=; b=2TiaD9wZn/tmg+bkwQITCGx3iOLS9uQkek5A7KgV7yL8ds2KbJIsP4aOpz4VOBnXdo+fdy lffwBdeI/Pq2eKnmP+GBzyVMTmM2fqP2LsONK1YYKuHjsHTI9tjw0ianj2zOtxBYwDiX5a YREhS5mlW/q8gNRz5hqKYNfKbV3E7t4YezWRkODgxugzOrAiRUKXrFx4xuPiz7cM5nfpMV 3vRn55VeYzLSIA3U0IeL2pQuiB+4UqLmEKLwbpPPOhFCqLRrSbTr1XrPmwzylncfuTU5Gg DinPIAQOtJo5kGjojGRTcVmzA6qDYU+DhkP+8JszgeCpHYRxaqMWLFDtuIGFig== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1682751787; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=SDaA+xUryUCZ8I28N9aEWarm2HCgNFHRBUqq4ZipNkw=; b=dx3FTou7JcyvSTYo/IWVXqQGwAgfGNHc79nquHXcwz8GGbKYbTQAV5Pys2EwAhqnZ79apd AeT0lRM3+F9AnVCw== From: "tip-bot2 for Sebastian Andrzej Siewior" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: locking/core] locking/rwbase: Mitigate indefinite writer starvation Cc: Mel Gorman , Sebastian Andrzej Siewior , Thomas Gleixner , Ingo Molnar , Linus Torvalds , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <877cwbq4cq.ffs@tglx> References: <877cwbq4cq.ffs@tglx> MIME-Version: 1.0 Message-ID: <168275178708.404.13683665200717626496.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the locking/core branch of tip: Commit-ID: 0abdb96fc6862681b926a0686012820229fab450 Gitweb: https://git.kernel.org/tip/0abdb96fc6862681b926a068601282022= 9fab450 Author: Sebastian Andrzej Siewior AuthorDate: Tue, 21 Mar 2023 17:11:40 +01:00 Committer: Ingo Molnar CommitterDate: Sat, 29 Apr 2023 08:51:08 +02:00 locking/rwbase: Mitigate indefinite writer starvation On PREEMPT_RT, rw_semaphore and rwlock_t locks are unfair to writers. Readers can indefinitely acquire the lock unless the writer fully acquired the lock, which might never happen if there is always a reader in the critical section owning the lock. Mel Gorman reported that since LTP-20220121 the dio_truncate test case went from having 1 reader to having 16 readers and that number of readers is sufficient to prevent the down_write ever succeeding while readers exist. Eventually the test is killed after 30 minutes as a failure. Mel proposed a timeout to limit how long a writer can be blocked until the reader is forced into the slowpath. Thomas argued that there is no added value by providing this timeout. From a PREEMPT_RT point of view, there are no critical rw_semaphore or rwlock_t locks left where the reader must be preferred. Mitigate indefinite writer starvation by forcing the READER into the slowpath once the WRITER attempts to acquire the lock. Reported-by: Mel Gorman Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Thomas Gleixner Signed-off-by: Ingo Molnar Acked-by: Mel Gorman Link: https://lore.kernel.org/877cwbq4cq.ffs@tglx Link: https://lore.kernel.org/r/20230321161140.HMcQEhHb@linutronix.de Cc: Linus Torvalds --- kernel/locking/rwbase_rt.c | 9 --------- 1 file changed, 9 deletions(-) diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c index c201aad..25ec023 100644 --- a/kernel/locking/rwbase_rt.c +++ b/kernel/locking/rwbase_rt.c @@ -72,15 +72,6 @@ static int __sched __rwbase_read_lock(struct rwbase_rt *= rwb, int ret; =20 raw_spin_lock_irq(&rtm->wait_lock); - /* - * Allow readers, as long as the writer has not completely - * acquired the semaphore for write. - */ - if (atomic_read(&rwb->readers) !=3D WRITER_BIAS) { - atomic_inc(&rwb->readers); - raw_spin_unlock_irq(&rtm->wait_lock); - return 0; - } =20 /* * Call into the slow lock path with the rtmutex->wait_lock