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 25CAEC6FD18 for ; Wed, 29 Mar 2023 03:46:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229814AbjC2DqB (ORCPT ); Tue, 28 Mar 2023 23:46:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37306 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229690AbjC2Dp6 (ORCPT ); Tue, 28 Mar 2023 23:45:58 -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 583D926AE for ; Tue, 28 Mar 2023 20:45:12 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1680061511; h=from:from: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=aSDDiMH0p9WcBAlTXqJi3wXATdVEQgYiEh/ApzzCZ+E=; b=FkqbWys/0iE8s5f3fHsHponQNEz4O62HJ/2oS78VaeHoyCB8yN5mud7b8/r0yInbcZKTxU q7jmJ+kRfDGrtSKfDSvXBlmqjCe/5aJZpBgVafEdRE/vkCdSoPFnz4bUKAasyNMUHttq+6 hRoo7w0NhomU88WVSmOGd9+CJzinE3Q= 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-160-L24i7gjiMWyGwUQzQL2r2w-1; Tue, 28 Mar 2023 23:45:07 -0400 X-MC-Unique: L24i7gjiMWyGwUQzQL2r2w-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id ED8061C041A0; Wed, 29 Mar 2023 03:45:06 +0000 (UTC) Received: from [10.22.18.156] (unknown [10.22.18.156]) by smtp.corp.redhat.com (Postfix) with ESMTP id AFD92492C3E; Wed, 29 Mar 2023 03:45:06 +0000 (UTC) Message-ID: <8b0b7f01-c286-0650-9600-c66467569e93@redhat.com> Date: Tue, 28 Mar 2023 23:45:06 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: [PATCH v2 8/8] locking/rwsem: Restore old write lock slow path behavior Content-Language: en-US To: Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Boqun Feng , linux-kernel@vger.kernel.org References: <20230327202413.1955856-1-longman@redhat.com> <20230327202413.1955856-9-longman@redhat.com> <20230328140259.GF4253@hirez.programming.kicks-ass.net> From: Waiman Long In-Reply-To: <20230328140259.GF4253@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/28/23 10:02, Peter Zijlstra wrote: > On Mon, Mar 27, 2023 at 04:24:13PM -0400, Waiman Long wrote: > >> kernel/locking/rwsem.c | 34 +++++++++++++++++++++++++++++++--- >> 1 file changed, 31 insertions(+), 3 deletions(-) >> >> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c >> index 7bd26e64827a..cf9dc1e250e0 100644 >> --- a/kernel/locking/rwsem.c >> +++ b/kernel/locking/rwsem.c >> @@ -426,6 +426,7 @@ rwsem_waiter_wake(struct rwsem_waiter *waiter, struct wake_q_head *wake_q) >> static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, >> struct rwsem_waiter *waiter) >> { >> + bool first = (rwsem_first_waiter(sem) == waiter); >> long count, new; >> >> lockdep_assert_held(&sem->wait_lock); >> @@ -434,6 +435,9 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, >> do { >> new = count; >> >> + if (!first && (count & (RWSEM_FLAG_HANDOFF | RWSEM_LOCK_MASK))) >> + return false; >> + >> if (count & RWSEM_LOCK_MASK) { >> /* >> * A waiter (first or not) can set the handoff bit > I couldn't immediately make sense of the above, and I think there's case > where not-first would still want to set handoff you're missing. It is possible to do that, but we need a minor change to make sure that you set the handoff_set flag of the first waiter instead of the current waiter which is what the current rwsem code is doing. Other than that, I think the change is OK, though I need to take a further look into it tomorrow as it is now late for me too. Cheers, longman > > After a few detours, I ended up with the below; does that make sense or > did I just make a bigger mess? (entirely possible due to insufficient > sleep etc..). > > > --- a/kernel/locking/rwsem.c > +++ b/kernel/locking/rwsem.c > @@ -426,12 +426,27 @@ rwsem_waiter_wake(struct rwsem_waiter *w > static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, > struct rwsem_waiter *waiter) > { > + bool first = (rwsem_first_waiter(sem) == waiter); > long count, new; > > lockdep_assert_held(&sem->wait_lock); > > count = atomic_long_read(&sem->count); > do { > + /* > + * first handoff > + * > + * 0 0 | take > + * 0 1 | not take > + * 1 0 | take > + * 1 1 | take > + * > + */ > + bool handoff = count & RWSEM_FLAG_HANDOFF; > + > + if (!first && handoff) > + return false; > + > new = count; > > if (count & RWSEM_LOCK_MASK) { > @@ -440,7 +455,7 @@ static inline bool rwsem_try_write_lock( > * if it is an RT task or wait in the wait queue > * for too long. > */ > - if ((count & RWSEM_FLAG_HANDOFF) || > + if (handoff || > (!rt_task(waiter->task) && > !time_after(jiffies, waiter->timeout))) > return false; > @@ -501,11 +516,19 @@ static void rwsem_writer_wake(struct rw_ > */ > list_del(&waiter->list); > atomic_long_set(&sem->owner, (long)waiter->task); > - > - } else if (!rwsem_try_write_lock(sem, waiter)) > + rwsem_waiter_wake(waiter, wake_q); > return; > + } > > - rwsem_waiter_wake(waiter, wake_q); > + /* > + * Mark writer at the front of the queue for wakeup. > + * > + * Until the task is actually awoken later by the caller, other writers > + * are able to steal it. Readers, on the other hand, will block as they > + * will notice the queued writer. > + */ > + wake_q_add(wake_q, waiter->task); > + lockevent_inc(rwsem_wake_writer); > } > > static void rwsem_reader_wake(struct rw_semaphore *sem, > @@ -1038,6 +1061,25 @@ rwsem_waiter_wait(struct rw_semaphore *s > /* Matches rwsem_waiter_wake()'s smp_store_release(). */ > break; > } > + if (!reader) { > + /* > + * If rwsem_writer_wake() did not take the lock, we must > + * rwsem_try_write_lock() here. > + */ > + raw_spin_lock_irq(&sem->wait_lock); > + if (waiter->task && rwsem_try_write_lock(sem, waiter)) { > + waiter->task = NULL; > + raw_spin_unlock_irq(&sem->wait_lock); > + break; > + } > + raw_spin_unlock_irq(&sem->wait_lock); > + > + if (waiter->handoff_set) > + rwsem_spin_on_owner(sem); > + > + if (!smp_load_acquire(&waiter->task)) > + break; > + } > if (signal_pending_state(state, current)) { > raw_spin_lock_irq(&sem->wait_lock); > if (waiter->task) >