From nobody Tue Apr 7 08:08:03 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 738BAC433FE for ; Wed, 12 Oct 2022 13:34:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229689AbiJLNeG (ORCPT ); Wed, 12 Oct 2022 09:34:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40112 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229638AbiJLNd5 (ORCPT ); Wed, 12 Oct 2022 09:33:57 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D3F45CF868 for ; Wed, 12 Oct 2022 06:33:55 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1665581635; 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: in-reply-to:in-reply-to:references:references; bh=k7o9ogdwYjPmHnjavjS1jzPX8BieWsdsKWNkb3se7Tg=; b=g0iD7UVVQMN/CLcSOjsdw2205Sh/BdvnuNpOK7h/hSzLPwByFlYIa48dJev+J7LIN+mMm4 cy/qDpo/ZeD1X9OCO3Eu+iJ7loRPdy87CKZhhSbwGS3n8CpPy3nBeu2FT7+95IGe1f1WHH gA5KLk/RcQUb+UkskNUhzy2FIlqEv3o= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-17-OMyTEN68OJeZZTXua95jEg-1; Wed, 12 Oct 2022 09:33:49 -0400 X-MC-Unique: OMyTEN68OJeZZTXua95jEg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 97D633817962; Wed, 12 Oct 2022 13:33:48 +0000 (UTC) Received: from llong.com (unknown [10.22.33.120]) by smtp.corp.redhat.com (Postfix) with ESMTP id 30CD6112131E; Wed, 12 Oct 2022 13:33:48 +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 , =?UTF-8?q?Ting11=20Wang=20=E7=8E=8B=E5=A9=B7?= , Waiman Long Subject: [PATCH v2 1/2] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath Date: Wed, 12 Oct 2022 09:33:32 -0400 Message-Id: <20221012133333.1265281-2-longman@redhat.com> In-Reply-To: <20221012133333.1265281-1-longman@redhat.com> References: <20221012133333.1265281-1-longman@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" 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 !=3D 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 consist= ent") 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 44873594de03..3839b38608da 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -636,6 +636,11 @@ static inline bool rwsem_try_write_lock(struct rw_sema= phore *sem, new =3D count; =20 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; @@ -651,11 +656,13 @@ static inline bool rwsem_try_write_lock(struct rw_sem= aphore *sem, } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new)); =20 /* - * 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 =3D true; + if (waiter =3D=3D first) + waiter->handoff_set =3D true; lockevent_inc(rwsem_wlock_handoff); return false; } --=20 2.31.1