From mboxrd@z Thu Jan 1 00:00:00 1970 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 97452C433F5 for ; Thu, 29 Sep 2022 18:04:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231314AbiI2SEt (ORCPT ); Thu, 29 Sep 2022 14:04:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46612 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235128AbiI2SEo (ORCPT ); Thu, 29 Sep 2022 14:04:44 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 311EC5FDC for ; Thu, 29 Sep 2022 11:04:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1664474682; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=UR5Zd9ORHHKbZxsxJNmBhTXsgUFFdrnVVqoQYsLLscc=; b=JTH4ZOU2qBfpbN5OFzHSBgNmb6Dy8lM+gHSIkilamd42Y75mFi6foHlT3SUMV6aoQI10Qe EEJHDvq/MN4JP7zetVD+r09Y7vVaFGxXQhlrp19EkjobufcImNUVeQyOSt44ZTp+vASjAD KuWkVQ5VmV7C3Xe9Y/5jfwY99hgoYO0= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-128-epj_Yl07OUGkYeqK3TfBdA-1; Thu, 29 Sep 2022 14:04:38 -0400 X-MC-Unique: epj_Yl07OUGkYeqK3TfBdA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 7DADD862FDC; Thu, 29 Sep 2022 18:04:38 +0000 (UTC) Received: from llong.com (unknown [10.22.8.248]) by smtp.corp.redhat.com (Postfix) with ESMTP id E050540C6EC2; Thu, 29 Sep 2022 18:04:37 +0000 (UTC) From: Waiman Long To: Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng Cc: linux-kernel@vger.kernel.org, john.p.donnelly@oracle.com, Hillf Danton , Mukesh Ojha , Waiman Long Subject: [PATCH] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath Date: Thu, 29 Sep 2022 14:04:13 -0400 Message-Id: <20220929180413.107374-1-longman@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org A non-first waiter can potentially spin in the for loop of rwsem_down_write_slowpath() without sleeping but fail to acquire the lock even if the rwsem is free if the following sequence happens: Non-first waiter First waiter Lock holder ---------------- ------------ ----------- Acquire wait_lock rwsem_try_write_lock(): Set handoff bit if RT or wait too long Set waiter->handoff_set Release wait_lock Acquire wait_lock Inherit waiter->handoff_set Release wait_lock Clear owner Release lock if (waiter.handoff_set) { rwsem_spin_on_owner((); if (OWNER_NULL) goto trylock_again; } trylock_again: Acquire wait_lock rwsem_try_write_lock(): if (first->handoff_set && (waiter != first)) return false; Release wait_lock It is especially problematic if the non-first waiter is an RT task and it is running on the same CPU as the first waiter as this can lead to live lock. Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more consistent") Signed-off-by: Waiman Long --- kernel/locking/rwsem.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-) diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index 65f0262f635e..ad676e99e0b3 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -628,6 +628,11 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, new = count; if (count & RWSEM_LOCK_MASK) { + /* + * A waiter (first or not) can set the handoff bit + * if it is an RT task or wait in the wait queue + * for too long. + */ if (has_handoff || (!rt_task(waiter->task) && !time_after(jiffies, waiter->timeout))) return false; @@ -643,11 +648,13 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new)); /* - * We have either acquired the lock with handoff bit cleared or - * set the handoff bit. + * We have either acquired the lock with handoff bit cleared or set + * the handoff bit. Only the first waiter can have its handoff_set + * set here to enable optimistic spinning in slowpath loop. */ if (new & RWSEM_FLAG_HANDOFF) { - waiter->handoff_set = true; + if (waiter == first) + waiter->handoff_set = true; lockevent_inc(rwsem_wlock_handoff); return false; } -- 2.31.1