From nobody Sun Feb 8 14:56:55 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 A0BD5C6FD1C for ; Wed, 22 Mar 2023 22:26:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229927AbjCVW0M (ORCPT ); Wed, 22 Mar 2023 18:26:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49032 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229614AbjCVW0K (ORCPT ); Wed, 22 Mar 2023 18:26:10 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2201D23A75; Wed, 22 Mar 2023 15:26:07 -0700 (PDT) Date: Wed, 22 Mar 2023 22:26:04 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1679523965; 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=Lonwx3OOvND6DSGV5w4ARUSC+QoyLxxKYdl6wMg0BrA=; b=DQIPeMyzjf47/vwhpwavHxHkar0zLrxOFRMKGJPOEwFhDeNruyVGMYjYd4nnPUXfzYnjz3 DTPLJ3vzx9AvtSSSDo6m4BJZLKzOQod745fmMhuNd9MyBaZfm1VgecwHmY2+uNqDDdLyWm IaMEcD8DJktVBSs9vIajnhiEGi10ajtMkSjSdmpoyh8zaYCqJGhTt1rk9n+UFq4pUbQYaf kvL7yBtYmtN+t/MW7ioeUMkwDwkLGeUNlFDKxfRUaKB/QArMl6vtdhGRpnhhSkZfhq8EnL BZXQUOnN4PuRXLVJewMLTjGMgly7lvmx74seC4uh9AoJeSpptHbVuPA8bT96eQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1679523965; 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=Lonwx3OOvND6DSGV5w4ARUSC+QoyLxxKYdl6wMg0BrA=; b=m1uMKNTln/b70+QVaOSRsOkfwH7aZZfkzfGaET+sCGniCdXDicZJ0iYhTD3IFB+Fwkori6 aFoNq8VE0x1YbdAw== 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 , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <877cwbq4cq.ffs@tglx> References: <877cwbq4cq.ffs@tglx> MIME-Version: 1.0 Message-ID: <167952396475.5837.11598351113694705444.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: 9d29d8e36cd6448966d83ab1acf946a2df3c2833 Gitweb: https://git.kernel.org/tip/9d29d8e36cd6448966d83ab1acf946a2d= f3c2833 Author: Sebastian Andrzej Siewior AuthorDate: Tue, 21 Mar 2023 17:11:40 +01:00 Committer: Thomas Gleixner CommitterDate: Wed, 22 Mar 2023 23:17:38 +01:00 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 Acked-by: Mel Gorman Link: https://lore.kernel.org/877cwbq4cq.ffs@tglx Link: https://lore.kernel.org/r/20230321161140.HMcQEhHb@linutronix.de --- 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