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 X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C988EC4332E for ; Thu, 19 Mar 2020 00:33:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8E42C20773 for ; Thu, 19 Mar 2020 00:33:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="vKjz15ir" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726796AbgCSAd4 (ORCPT ); Wed, 18 Mar 2020 20:33:56 -0400 Received: from mail-qt1-f196.google.com ([209.85.160.196]:35402 "EHLO mail-qt1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727163AbgCSAdz (ORCPT ); Wed, 18 Mar 2020 20:33:55 -0400 Received: by mail-qt1-f196.google.com with SMTP id v15so400397qto.2 for ; Wed, 18 Mar 2020 17:33:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=cHl5FEDU/in/tKhyh8KADSkZ8t44HVVU6qSIiQF7HDk=; b=vKjz15irrUHQOxGKx3YJZSYaREZiLBQ5dkdKASVNi/DUnapYI1oeMp9PSbchikzKPl RL8eE9BxcoQ9gT2Z+96oGA/M+zvf4M7Ww0jlnMdeOCV7wPsoaRRDYZ+t1YRHJBhYvxmw JQHaOq18/lt39V3Rogoc35Lu1FpkxEJUwdFM4= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=cHl5FEDU/in/tKhyh8KADSkZ8t44HVVU6qSIiQF7HDk=; b=d2W+DXK7DeCv8Gu0lEuM7thuCeSNDxJUcLm9AwvPl4gvzAyLQbKnIvgCv8ctUoOryB yU6CoFqu4PgM0dz9Kfx9TjboRjH7n6woLMfbrl3syMsT+rRI33y83gN2mkp8WGj3DI3M HHatOh4pIpNMhO+HyxX+7H/35FDh6FHZ7dBMsB6mLOkblSTUqiLhCHpX+fssU4RZgqkG s4y2lsSgbm5qJ0MHm5A8vR0CZIEPk1wxhjlC/Udpb/jTs8mvK9d2iJjdxkskdw8NHlFC EZi40sqDuNBVDk5bIHOHMWJ/QXSmkOoaobiNSi3GoxUkkR28ApzDxp5UGs9ZbeVwPS0N Eh9w== X-Gm-Message-State: ANhLgQ3o+BcS1wd00HRg3hGMXjXSxvRF7/Rn+z2WTH7rmRLiJMB1Ms1v MKrij6eNEfVlsBLJPd/fXFEIWw== X-Google-Smtp-Source: ADFU+vte9vg5yrKnsJYwQUFLJMp35Yp0oNpQIim2wyXmoa5rkeGpiqYzRFosPkhMiCPa11iAMkqA+g== X-Received: by 2002:ac8:1762:: with SMTP id u31mr389327qtk.359.1584578032733; Wed, 18 Mar 2020 17:33:52 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id f13sm456312qkm.19.2020.03.18.17.33.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 18 Mar 2020 17:33:52 -0700 (PDT) Date: Wed, 18 Mar 2020 20:33:51 -0400 From: Joel Fernandes To: Thomas Gleixner Cc: LKML , Peter Zijlstra , Linus Torvalds , Ingo Molnar , Will Deacon , "Paul E . McKenney" , Steven Rostedt , Randy Dunlap , Arnd Bergmann , Sebastian Andrzej Siewior , Logan Gunthorpe , Kurt Schwemmer , Bjorn Helgaas , linux-pci@vger.kernel.org, Felipe Balbi , Greg Kroah-Hartman , linux-usb@vger.kernel.org, Kalle Valo , "David S. Miller" , linux-wireless@vger.kernel.org, netdev@vger.kernel.org, Oleg Nesterov , Davidlohr Bueso , Michael Ellerman , linuxppc-dev@lists.ozlabs.org Subject: Re: [patch V2 11/15] completion: Use simple wait queues Message-ID: <20200319003351.GA211584@google.com> References: <20200318204302.693307984@linutronix.de> <20200318204408.521507446@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200318204408.521507446@linutronix.de> User-Agent: Mutt/1.12.2 (2019-09-21) Sender: linux-pci-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org Hi Thomas, On Wed, Mar 18, 2020 at 09:43:13PM +0100, Thomas Gleixner wrote: > From: Thomas Gleixner > > completion uses a wait_queue_head_t to enqueue waiters. > > wait_queue_head_t contains a spinlock_t to protect the list of waiters > which excludes it from being used in truly atomic context on a PREEMPT_RT > enabled kernel. > > The spinlock in the wait queue head cannot be replaced by a raw_spinlock > because: > > - wait queues can have custom wakeup callbacks, which acquire other > spinlock_t locks and have potentially long execution times Cool, makes sense. > - wake_up() walks an unbounded number of list entries during the wake up > and may wake an unbounded number of waiters. Just to clarify here, wake_up() will really wake up just 1 waiter if all the waiters on the queue are exclusive right? So in such scenario at least, the "unbounded number of waiters" would not be an issue if everything waiting was exclusive and waitqueue with wake_up() was used. Please correct me if I'm wrong about that though. So the main reasons to avoid waitqueue in favor of swait (as you mentioned) would be the sleep-while-atomic issue in truly atomic context on RT, and the fact that callbacks can take a long time. > > For simplicity and performance reasons complete() should be usable on > PREEMPT_RT enabled kernels. > > completions do not use custom wakeup callbacks and are usually single > waiter, except for a few corner cases. > > Replace the wait queue in the completion with a simple wait queue (swait), > which uses a raw_spinlock_t for protecting the waiter list and therefore is > safe to use inside truly atomic regions on PREEMPT_RT. > > There is no semantical or functional change: > > - completions use the exclusive wait mode which is what swait provides > > - complete() wakes one exclusive waiter > > - complete_all() wakes all waiters while holding the lock which protects > the wait queue against newly incoming waiters. The conversion to swait > preserves this behaviour. > > complete_all() might cause unbound latencies with a large number of waiters > being woken at once, but most complete_all() usage sites are either in > testing or initialization code or have only a really small number of > concurrent waiters which for now does not cause a latency problem. Keep it > simple for now. > > The fixup of the warning check in the USB gadget driver is just a straight > forward conversion of the lockless waiter check from one waitqueue type to > the other. > > Signed-off-by: Thomas Gleixner > Cc: Arnd Bergmann Reviewed-by: Joel Fernandes (Google) thanks, - Joel > --- > V2: Split out the orinoco and usb gadget parts and amended change log > --- > drivers/usb/gadget/function/f_fs.c | 2 +- > include/linux/completion.h | 8 ++++---- > kernel/sched/completion.c | 36 +++++++++++++++++++----------------- > 3 files changed, 24 insertions(+), 22 deletions(-) > > --- a/drivers/usb/gadget/function/f_fs.c > +++ b/drivers/usb/gadget/function/f_fs.c > @@ -1703,7 +1703,7 @@ static void ffs_data_put(struct ffs_data > pr_info("%s(): freeing\n", __func__); > ffs_data_clear(ffs); > BUG_ON(waitqueue_active(&ffs->ev.waitq) || > - waitqueue_active(&ffs->ep0req_completion.wait) || > + swait_active(&ffs->ep0req_completion.wait) || > waitqueue_active(&ffs->wait)); > destroy_workqueue(ffs->io_completion_wq); > kfree(ffs->dev_name); > --- a/include/linux/completion.h > +++ b/include/linux/completion.h > @@ -9,7 +9,7 @@ > * See kernel/sched/completion.c for details. > */ > > -#include > +#include > > /* > * struct completion - structure used to maintain state for a "completion" > @@ -25,7 +25,7 @@ > */ > struct completion { > unsigned int done; > - wait_queue_head_t wait; > + struct swait_queue_head wait; > }; > > #define init_completion_map(x, m) __init_completion(x) > @@ -34,7 +34,7 @@ static inline void complete_acquire(stru > static inline void complete_release(struct completion *x) {} > > #define COMPLETION_INITIALIZER(work) \ > - { 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) } > + { 0, __SWAIT_QUEUE_HEAD_INITIALIZER((work).wait) } > > #define COMPLETION_INITIALIZER_ONSTACK_MAP(work, map) \ > (*({ init_completion_map(&(work), &(map)); &(work); })) > @@ -85,7 +85,7 @@ static inline void complete_release(stru > static inline void __init_completion(struct completion *x) > { > x->done = 0; > - init_waitqueue_head(&x->wait); > + init_swait_queue_head(&x->wait); > } > > /** > --- a/kernel/sched/completion.c > +++ b/kernel/sched/completion.c > @@ -29,12 +29,12 @@ void complete(struct completion *x) > { > unsigned long flags; > > - spin_lock_irqsave(&x->wait.lock, flags); > + raw_spin_lock_irqsave(&x->wait.lock, flags); > > if (x->done != UINT_MAX) > x->done++; > - __wake_up_locked(&x->wait, TASK_NORMAL, 1); > - spin_unlock_irqrestore(&x->wait.lock, flags); > + swake_up_locked(&x->wait); > + raw_spin_unlock_irqrestore(&x->wait.lock, flags); > } > EXPORT_SYMBOL(complete); > > @@ -58,10 +58,12 @@ void complete_all(struct completion *x) > { > unsigned long flags; > > - spin_lock_irqsave(&x->wait.lock, flags); > + WARN_ON(irqs_disabled()); > + > + raw_spin_lock_irqsave(&x->wait.lock, flags); > x->done = UINT_MAX; > - __wake_up_locked(&x->wait, TASK_NORMAL, 0); > - spin_unlock_irqrestore(&x->wait.lock, flags); > + swake_up_all_locked(&x->wait); > + raw_spin_unlock_irqrestore(&x->wait.lock, flags); > } > EXPORT_SYMBOL(complete_all); > > @@ -70,20 +72,20 @@ do_wait_for_common(struct completion *x, > long (*action)(long), long timeout, int state) > { > if (!x->done) { > - DECLARE_WAITQUEUE(wait, current); > + DECLARE_SWAITQUEUE(wait); > > - __add_wait_queue_entry_tail_exclusive(&x->wait, &wait); > do { > if (signal_pending_state(state, current)) { > timeout = -ERESTARTSYS; > break; > } > + __prepare_to_swait(&x->wait, &wait); > __set_current_state(state); > - spin_unlock_irq(&x->wait.lock); > + raw_spin_unlock_irq(&x->wait.lock); > timeout = action(timeout); > - spin_lock_irq(&x->wait.lock); > + raw_spin_lock_irq(&x->wait.lock); > } while (!x->done && timeout); > - __remove_wait_queue(&x->wait, &wait); > + __finish_swait(&x->wait, &wait); > if (!x->done) > return timeout; > } > @@ -100,9 +102,9 @@ static inline long __sched > > complete_acquire(x); > > - spin_lock_irq(&x->wait.lock); > + raw_spin_lock_irq(&x->wait.lock); > timeout = do_wait_for_common(x, action, timeout, state); > - spin_unlock_irq(&x->wait.lock); > + raw_spin_unlock_irq(&x->wait.lock); > > complete_release(x); > > @@ -291,12 +293,12 @@ bool try_wait_for_completion(struct comp > if (!READ_ONCE(x->done)) > return false; > > - spin_lock_irqsave(&x->wait.lock, flags); > + raw_spin_lock_irqsave(&x->wait.lock, flags); > if (!x->done) > ret = false; > else if (x->done != UINT_MAX) > x->done--; > - spin_unlock_irqrestore(&x->wait.lock, flags); > + raw_spin_unlock_irqrestore(&x->wait.lock, flags); > return ret; > } > EXPORT_SYMBOL(try_wait_for_completion); > @@ -322,8 +324,8 @@ bool completion_done(struct completion * > * otherwise we can end up freeing the completion before complete() > * is done referencing it. > */ > - spin_lock_irqsave(&x->wait.lock, flags); > - spin_unlock_irqrestore(&x->wait.lock, flags); > + raw_spin_lock_irqsave(&x->wait.lock, flags); > + raw_spin_unlock_irqrestore(&x->wait.lock, flags); > return true; > } > EXPORT_SYMBOL(completion_done); >