From nobody Thu Feb 12 03:00:11 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 D26A7C77B60 for ; Sat, 29 Apr 2023 07:18:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231266AbjD2HSa (ORCPT ); Sat, 29 Apr 2023 03:18:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47942 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230382AbjD2HSM (ORCPT ); Sat, 29 Apr 2023 03:18:12 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 201711FE3; Sat, 29 Apr 2023 00:18:11 -0700 (PDT) Date: Sat, 29 Apr 2023 07:18:09 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1682752689; 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=L4mRMu/XBMlNa2//UB0CSTDfD3HVMWUOiMwsBdt0pwQ=; b=JhKfq7WkyW3MLuWGG1zrUoFZ1bxFELuQfnYk6xJ2nOEXliw6tWu1iqgvZ3BUpTrWp7zZ+U kQOoimrglTKDtaWLZrNgkQFP27jmc5XDR+AGNra+GvoiWoyIYFD/4NPO5QcOUbv7V+fvYR f2/RuUNXMwBrQccOSyqjX8Ep2o0pwY4jiTLD4miZ91EH5wRk1Dg1LGGkTgFv9ZQorDr7Hq 9cKx01EibFhYa0CD1Npig88l+OQOMPLo0pg3+nojw8Ua7DE2pqArOCphPmFyyUc6mQ9K2A KXEv2p1cRDmkWz5o2Al3/uN2WgtAZnBd/uTGvYLWdL85rzzFTMsLZLk5djdpFg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1682752689; 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=L4mRMu/XBMlNa2//UB0CSTDfD3HVMWUOiMwsBdt0pwQ=; b=rEWEhDey5/l+m3yDULeOZHMbQ8XrDylpzvvE4XejFPOhHA+PYtmXlczThxPABjZJ60vwfG jJXSyeN0yJHQoeDw== 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: <168275268912.404.17554159478151480326.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: 286deb7ec03d941664ac3ffaff58814b454adf65 Gitweb: https://git.kernel.org/tip/286deb7ec03d941664ac3ffaff58814b4= 54adf65 Author: Sebastian Andrzej Siewior AuthorDate: Tue, 21 Mar 2023 17:11:40 +01:00 Committer: Ingo Molnar CommitterDate: Sat, 29 Apr 2023 09:08:52 +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